FN-5755: harden mission validation recovery flow

Ensure mission feature assertions recover and validate consistently across startup and periodic maintenance.

- add FN-5755 changeset and AGENTS reliability backstop note
- document canonical zero-assertion auto-pass and assertion validation lifecycle updates in missions docs
- expand mission execution loop and reliability interaction tests for startup recovery, periodic replay, and idempotency
- wire self-healing maintenance to replay active mission validation recovery via runtime callback

Files changed:
 .../fn-5755-mission-validation-end-to-end.md       |  5 ++
 AGENTS.md                                          |  1 +
 docs/missions-completion-contract.md               | 15 ++--
 docs/missions.md                                   |  4 +-
 .../src/__tests__/mission-execution-loop.test.ts   | 88 +++++++++++++++++++++-
 .../mission-validation-trigger-gap.test.ts         | 42 ++++++++++-
 packages/engine/src/runtimes/in-process-runtime.ts |  6 ++
 packages/engine/src/self-healing.ts                | 11 +++
 8 files changed, 159 insertions(+), 13 deletions(-)

Fusion-Task-Id: FN-5755

Fusion-Task-Lineage: e54ac4dc-7b8c-4fc9-8cd6-18702708546f

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

@@ -732,6 +732,12 @@ export class InProcessRuntime
         getActiveMergeTaskId: () => this.activeMergeTaskIdProvider?.() ?? null,
         leaseManager: this.leaseManager,
         hasActiveAgentExecution: (agentId: string) => this.heartbeatMonitor?.getTrackedAgents().includes(agentId) ?? false,
+        recoverActiveMissionValidations: async () => {
+          if (!this.missionExecutionLoop) {
+            return { recoveredCount: 0 };
+          }
+          return this.missionExecutionLoop.recoverActiveMissions();
+        },
         reconcileAllMissionFeatures: async () => this.scheduler.reconcileAllMissionFeatures(),
         chatStore: this.chatStore,
         messageStore: this.messageStore,