test(coverage): add Behave BDD tests for 8 under-covered modules

Added targeted Behave BDD feature files and step definitions to improve
unit test coverage for:

- decision_service.py: Full coverage of all 7 service methods (18 scenarios)
- plan_apply_service.py: Branch coverage for handle_merge_failure (2 scenarios)
- plan_executor.py: Edge cases for rollback, checkpoint, and parse_steps (15 scenarios)
- cli/commands/plan.py: Uncovered region lines 1950-2273 (23 scenarios)
- repositories.py: Remaining missed branches and lines (14 scenarios)
- sandbox/checkpoint.py: Full coverage of CheckpointManager (26 scenarios)
- langgraph/bridge.py: Remaining uncovered lines and branches (10 scenarios)
- cli/commands/config.py: Safety net to maintain 100% coverage (42 scenarios)

Total: 150 new scenarios, 596 steps, all passing.

Also fixed a step definition collision in plan_lifecycle_coverage by renaming
"the delete result should be false" to "the plan delete result should be false".

ISSUES CLOSED: #475

features/decision_service_coverage.feature

@@ -0,0 +1,156 @@
+Feature: DecisionService application-layer coverage
+  As a developer
+  I want thorough unit tests for every DecisionService method
+  So that decision_service.py achieves full line and branch coverage
+
+  # All steps use the "dsvc-" prefix to avoid collisions with other step files.
+
+  # ------------------------------------------------------------------
+  # Constructor
+  # ------------------------------------------------------------------
+
+  Scenario: dsvc- constructor stores settings and unit_of_work
+    Given dsvc- a mock settings object and a mock UnitOfWork
+    When dsvc- I construct a DecisionService with those dependencies
+    Then dsvc- the service should store the settings
+    And dsvc- the service should store the unit_of_work
+    And dsvc- the service should have a bound structlog logger
+
+  # ------------------------------------------------------------------
+  # record_decision
+  # ------------------------------------------------------------------
+
+  Scenario: dsvc- record_decision persists and returns the decision
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- a sample root Decision object
+    When dsvc- I call record_decision with the sample decision
+    Then dsvc- the UoW transaction should have been entered
+    And dsvc- ctx.decisions.create should have been called with the decision
+    And dsvc- the returned decision should be the same object
+
+  Scenario: dsvc- record_decision logs info and debug messages
+    Given dsvc- a DecisionService with a mocked UnitOfWork and captured logger
+    And dsvc- a sample root Decision object
+    When dsvc- I call record_decision with the sample decision
+    Then dsvc- the logger should have recorded an info call with "recording_decision"
+    And dsvc- the logger should have recorded a debug call with "decision_recorded"
+
+  # ------------------------------------------------------------------
+  # get_decision
+  # ------------------------------------------------------------------
+
+  Scenario: dsvc- get_decision retrieves a decision by ID
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo get method returns a Decision
+    When dsvc- I call get_decision with a known ID
+    Then dsvc- ctx.decisions.get should have been called with the ID
+    And dsvc- the returned value should be the expected Decision
+
+  Scenario: dsvc- get_decision returns None when not found
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo get method returns None
+    When dsvc- I call get_decision with an unknown ID
+    Then dsvc- the returned value should be None
+
+  Scenario: dsvc- get_decision logs a debug message
+    Given dsvc- a DecisionService with a mocked UnitOfWork and captured logger
+    And dsvc- the mock repo get method returns a Decision
+    When dsvc- I call get_decision with a known ID
+    Then dsvc- the logger should have recorded a debug call with "getting_decision"
+
+  # ------------------------------------------------------------------
+  # get_decisions_for_plan
+  # ------------------------------------------------------------------
+
+  Scenario: dsvc- get_decisions_for_plan returns list of decisions
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo get_by_plan method returns 3 decisions
+    When dsvc- I call get_decisions_for_plan with a plan ID
+    Then dsvc- ctx.decisions.get_by_plan should have been called with the plan ID
+    And dsvc- the returned list should have 3 decisions
+
+  Scenario: dsvc- get_decisions_for_plan returns empty list when none exist
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo get_by_plan method returns 0 decisions
+    When dsvc- I call get_decisions_for_plan with a plan ID
+    Then dsvc- the returned list should have 0 decisions
+
+  Scenario: dsvc- get_decisions_for_plan logs a debug message
+    Given dsvc- a DecisionService with a mocked UnitOfWork and captured logger
+    And dsvc- the mock repo get_by_plan method returns 3 decisions
+    When dsvc- I call get_decisions_for_plan with a plan ID
+    Then dsvc- the logger should have recorded a debug call with "getting_decisions_for_plan"
+
+  # ------------------------------------------------------------------
+  # get_decision_tree
+  # ------------------------------------------------------------------
+
+  Scenario: dsvc- get_decision_tree returns BFS-ordered list
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo get_tree method returns 4 decisions
+    When dsvc- I call get_decision_tree with a root ID
+    Then dsvc- ctx.decisions.get_tree should have been called with the root ID
+    And dsvc- the returned tree list should have 4 decisions
+
+  Scenario: dsvc- get_decision_tree logs a debug message
+    Given dsvc- a DecisionService with a mocked UnitOfWork and captured logger
+    And dsvc- the mock repo get_tree method returns 4 decisions
+    When dsvc- I call get_decision_tree with a root ID
+    Then dsvc- the logger should have recorded a debug call with "getting_decision_tree"
+
+  # ------------------------------------------------------------------
+  # get_path_to_root
+  # ------------------------------------------------------------------
+
+  Scenario: dsvc- get_path_to_root returns leaf-to-root path
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo get_path_to_root method returns 3 decisions
+    When dsvc- I call get_path_to_root with a leaf ID
+    Then dsvc- ctx.decisions.get_path_to_root should have been called with the leaf ID
+    And dsvc- the returned path list should have 3 decisions
+
+  Scenario: dsvc- get_path_to_root logs a debug message
+    Given dsvc- a DecisionService with a mocked UnitOfWork and captured logger
+    And dsvc- the mock repo get_path_to_root method returns 3 decisions
+    When dsvc- I call get_path_to_root with a leaf ID
+    Then dsvc- the logger should have recorded a debug call with "getting_path_to_root"
+
+  # ------------------------------------------------------------------
+  # mark_superseded
+  # ------------------------------------------------------------------
+
+  Scenario: dsvc- mark_superseded updates and returns the decision
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo update_superseded_by method returns a superseded Decision
+    When dsvc- I call mark_superseded with old and new IDs
+    Then dsvc- ctx.decisions.update_superseded_by should have been called with both IDs
+    And dsvc- the returned decision should be the superseded one
+
+  Scenario: dsvc- mark_superseded logs info with both IDs
+    Given dsvc- a DecisionService with a mocked UnitOfWork and captured logger
+    And dsvc- the mock repo update_superseded_by method returns a superseded Decision
+    When dsvc- I call mark_superseded with old and new IDs
+    Then dsvc- the logger should have recorded an info call with "marking_superseded"
+
+  # ------------------------------------------------------------------
+  # list_by_type
+  # ------------------------------------------------------------------
+
+  Scenario: dsvc- list_by_type returns filtered decisions
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo list_by_type method returns 2 decisions
+    When dsvc- I call list_by_type with plan ID and type "strategy_choice"
+    Then dsvc- ctx.decisions.list_by_type should have been called with plan ID and type
+    And dsvc- the returned type list should have 2 decisions
+
+  Scenario: dsvc- list_by_type returns empty when no matches
+    Given dsvc- a DecisionService with a mocked UnitOfWork
+    And dsvc- the mock repo list_by_type method returns 0 decisions
+    When dsvc- I call list_by_type with plan ID and type "error_recovery"
+    Then dsvc- the returned type list should have 0 decisions
+
+  Scenario: dsvc- list_by_type logs a debug message
+    Given dsvc- a DecisionService with a mocked UnitOfWork and captured logger
+    And dsvc- the mock repo list_by_type method returns 2 decisions
+    When dsvc- I call list_by_type with plan ID and type "strategy_choice"
+    Then dsvc- the logger should have recorded a debug call with "listing_by_type"