fix(plan): implement end-to-end subplan execution orchestration

Replace the metadata-only subplan spawn with real child Plan domain object
creation. SubplanService.spawn() now creates full Plan instances with proper
PlanIdentity (linking parent_plan_id and root_plan_id), sets them to
PlanPhase.STRATEGIZE / ProcessingState.QUEUED, and returns them in the
SpawnResult.child_plans list for downstream lifecycle orchestration.

Key changes:
- SubplanService.spawn() now creates Plan domain objects for each spawn entry
  with complete PlanIdentity linking, inheriting the parent plan's namespace,
  actors, project links, definition_of_done, and access settings.
- SpawnResult dataclass extended with child_plans: list[Plan] field to carry
  the created child plans alongside the existing metadata and statuses.
- Parent plan's subplan_statuses list is updated with SubplanStatus entries
  tracking each child's lifecycle state.
- Fixed Pyright type errors: added missing definition_of_done, reusable,
  read_only, and server parameters to Plan and NamespacedName constructors.
- Removed @tdd_expected_fail tags from TDD test files since the bug is fixed.
- Added Behave scenarios and Robot integration tests verifying child plan
  creation, lifecycle phase/state, and parent tracking.

ISSUES CLOSED: #823

features/subplan_spawn_orchestration.feature

@@ -0,0 +1,44 @@
+@mock_only @subplan @orchestration
+Feature: Subplan spawn → execute → merge orchestration lifecycle
+  As a system orchestrating hierarchical plan execution
+  I want SubplanService.spawn() to create real child Plan domain objects
+  and wire them through the full strategize → execute → validate cycle
+  So that subplan execution is end-to-end orchestrated
+
+  Background:
+    Given a parent plan configured for subplan spawning
+    And valid spawn entries for the parent plan
+
+  Scenario: Spawn creates child Plans with correct parent-child identity
+    When I spawn subplans via SubplanService
+    Then the spawn result should contain child Plan domain objects
+    And each child Plan should have parent_plan_id set to the parent
+    And each child Plan root_plan_id should trace to the root plan
+
+  Scenario: Child plans inherit project links from parent
+    When I spawn subplans via SubplanService
+    Then each child plan should inherit the parent project links
+
+  Scenario: Parent plan blocks when child plans are pending
+    When I spawn subplans via SubplanService
+    Then the parent plan should have pending subplan statuses
+    And the parent plan subplan_statuses count should equal spawn count
+
+  Scenario: Child plan failure marks parent subplan status as errored
+    When I spawn subplans via SubplanService
+    And a child plan fails with error "ChildExecError: timeout"
+    Then the corresponding parent subplan status should be errored
+    And the parent subplan status error should contain "ChildExecError"
+
+  Scenario: Child plan completion updates parent subplan status
+    When I spawn subplans via SubplanService
+    And a child plan completes successfully with changeset summary "Added tests"
+    Then the corresponding parent subplan status should be complete
+    And the parent subplan status changeset summary should be "Added tests"
+
+  Scenario: Parallel spawn sets execution mode on result
+    Given a parent plan configured for parallel subplan spawning
+    And valid spawn entries for the parent plan
+    When I spawn subplans via SubplanService
+    Then the spawn result execution mode should be parallel
+    And the spawn result child plan count should match entries