FN-5754: re-triage stranded features in active mission slices

Ensure active mission slices recover stranded features by linking or auto-triaging them during scheduler and self-healing reconciliation.

- Extend mission feature reconciliation to auto-triage defined features in active slices when mission autopilot/auto-advance is enabled and no linked task can be found.
- Emit a new mission:stranded-feature-triaged audit event whenever stranded features are linked or triaged, and wire reconciliation into self-healing maintenance runs.
- Add regression coverage for scheduler and reliability interactions, and document the new backstop in AGENTS and mission docs.

Files changed:
 AGENTS.md                                          |   1 +
 docs/missions-completion-contract.md               |   1 +
 docs/missions.md                                   |   1 +
 .../mission-stranded-feature-retriage.test.ts      | 160 +++++++++++++++++++++
 packages/engine/src/__tests__/scheduler.test.ts    | 155 ++++++++++++++++++++
 packages/engine/src/run-audit.ts                   |   1 +
 packages/engine/src/runtimes/in-process-runtime.ts |   1 +
 packages/engine/src/scheduler.ts                   |  64 ++++++++-
 packages/engine/src/self-healing.ts                |  11 ++
 9 files changed, 394 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-5754

Fusion-Task-Lineage: 6b29754a-2fdf-4528-aa43-af294c4210df

packages/engine/src/runtimes/in-process-runtime.ts

@@ -732,6 +732,7 @@ export class InProcessRuntime
         getActiveMergeTaskId: () => this.activeMergeTaskIdProvider?.() ?? null,
         leaseManager: this.leaseManager,
         hasActiveAgentExecution: (agentId: string) => this.heartbeatMonitor?.getTrackedAgents().includes(agentId) ?? false,
+        reconcileAllMissionFeatures: async () => this.scheduler.reconcileAllMissionFeatures(),
         chatStore: this.chatStore,
         messageStore: this.messageStore,
         restartDurableAgentHeartbeat: async (agentId: string, context: { reason: string; attempt: number }) => {