feat(actor): add plan_subplan tool and decision emission

Add builtin/plan-subplan tool for strategy actors to emit SUBPLAN_SPAWN
or SUBPLAN_PARALLEL_SPAWN decisions when decomposing a plan into child
plans. Implements all acceptance criteria from issue #198:

- SubplanPayload (Pydantic) validates goal, resource_scopes/project_ref
  (at least one required), merge_strategy, max_parallel (1-50), parallel
  flag, dependencies, and context_view override.
- Defaults: merge_strategy=git_three_way, max_parallel=5, parallel=False,
  dependencies=[]. Omitted fields inherit sensible values automatically.
- make_plan_subplan_spec(decision_service=None) factory supports optional
  DecisionService injection for persistent decision recording.
- _build_rationale() generates human-readable rationale text (goal, scope,
  execution mode, dependencies, context_view) surfaced in plan explain.
- register_subplan_tool() added to tool/builtins/__init__.py for bulk
  registration. PLAN_SUBPLAN_SPEC exported as the default ready-to-use spec.
- Actor YAML example (examples/actors/strategy_with_subplan.yaml) with
  annotated serial and parallel spawn payload examples.
- Behave BDD: 20 scenarios, 70 steps covering validation, defaults,
  decision type, rationale, service injection, registry, and ToolRunner.
- Robot Framework: 9 smoke tests via robot/plan_subplan_tool.robot.
- ASV benchmarks: benchmarks/subplan_actor_tool_bench.py (5 suites).
- Coverage: 100% on subplan_tool.py. Lint, typecheck, and security clean.

ISSUES CLOSED: #198

examples/actors/strategy_with_subplan.yaml

@@ -0,0 +1,66 @@
+# Strategy Actor — Subplan Emission
+#
+# Demonstrates a strategy actor that uses builtin/plan-subplan to spawn
+# sequential and parallel child plans during the Strategize phase.
+#
+# The actor calls builtin/plan-subplan with the following payload:
+#   - goal:            What the child plan should accomplish
+#   - resource_scopes: Paths or resource IDs the child plan targets
+#   - parallel:        True → SUBPLAN_PARALLEL_SPAWN; False → SUBPLAN_SPAWN
+#   - merge_strategy:  How to merge child-plan results (default: git_three_way)
+#   - max_parallel:    Maximum concurrent subplans (default: 5)
+#   - dependencies:    Sibling subplan ULIDs this plan depends on
+
+name: local/strategy_with_subplan
+type: llm
+description: >
+  Strategy actor that decomposes a large plan into coordinated subplans
+  using the builtin/plan-subplan tool.
+version: "1.0"
+
+model: gpt-4-turbo
+context_view: strategist
+
+system_prompt: |
+  You are a strategy actor responsible for decomposing complex plans into
+  child subplans. Use the builtin/plan-subplan tool to emit subplan decisions.
+
+  For sequential work (one subplan at a time):
+    Call plan-subplan with parallel=false.
+
+  For parallel work (multiple subplans at once):
+    Call plan-subplan with parallel=true and list any dependencies.
+
+  Always include at least one resource_scope or project_ref.
+
+tools:
+  - builtin/plan-subplan
+
+memory:
+  enabled: false
+
+# ---------------------------------------------------------------------------
+# Example: Sequential subplan payload
+#
+# {
+#   "goal": "Refactor the authentication module",
+#   "plan_id": "01ARZ3NDEKTSV4RRFFQ69G5FAV",
+#   "sequence_number": 1,
+#   "resource_scopes": ["src/auth/", "tests/auth/"],
+#   "parallel": false,
+#   "merge_strategy": "git_three_way"
+# }
+#
+# Example: Parallel subplan payload with dependencies
+#
+# {
+#   "goal": "Run full test suite",
+#   "plan_id": "01ARZ3NDEKTSV4RRFFQ69G5FAV",
+#   "sequence_number": 2,
+#   "resource_scopes": ["tests/"],
+#   "parallel": true,
+#   "max_parallel": 3,
+#   "dependencies": ["01ARZ3NDEKTSV4RRFFQ69G5FAW"],
+#   "merge_strategy": "fail_on_conflict"
+# }
+# ---------------------------------------------------------------------------