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

robot/tdd_subplan_spawn_orchestration.robot

@@ -16,7 +16,7 @@ ${HELPER}    ${CURDIR}/helper_tdd_subplan_spawn_orchestration.py
 TDD Spawn Result Contains Child Plan Objects
     [Documentation]    Verify that spawn() returns actual child Plan domain objects,
     ...                not just SubplanStatus metadata.
-    [Tags]    tdd_expected_fail    tdd_bug    tdd_bug_823
+    [Tags]    tdd_bug    tdd_bug_823
     ${result}=    Run Process    ${PYTHON}    ${HELPER}    spawn-child-plans    cwd=${WORKSPACE}    timeout=30s
     Log    ${result.stdout}
     Log    ${result.stderr}
@@ -26,7 +26,7 @@ TDD Spawn Result Contains Child Plan Objects
 TDD Child Plans Enter Strategize Phase
     [Documentation]    Verify that spawned child plans enter the strategize phase
     ...                and have their lifecycle triggered.
-    [Tags]    tdd_expected_fail    tdd_bug    tdd_bug_823
+    [Tags]    tdd_bug    tdd_bug_823
     ${result}=    Run Process    ${PYTHON}    ${HELPER}    spawn-lifecycle    cwd=${WORKSPACE}    timeout=30s
     Log    ${result.stdout}
     Log    ${result.stderr}
@@ -36,7 +36,7 @@ TDD Child Plans Enter Strategize Phase
 TDD Parent Plan Tracks Child Status
     [Documentation]    Verify that spawn() attaches SubplanStatus entries to
     ...                the parent plan so lifecycle can be tracked.
-    [Tags]    tdd_expected_fail    tdd_bug    tdd_bug_823
+    [Tags]    tdd_bug    tdd_bug_823
     ${result}=    Run Process    ${PYTHON}    ${HELPER}    spawn-parent-tracking    cwd=${WORKSPACE}    timeout=30s
     Log    ${result.stdout}
     Log    ${result.stderr}