feat(FN-1257): add runContext audit trail for task mutations

- Add RunMutationContext type to track which agent run caused a mutation
- Thread runContext through TaskStore.logEntry, addComment, addSteeringComment, and pauseTask
- Propagate runContext from HeartbeatMonitor.executeHeartbeat to task store operations
- Propagate runContext from TaskExecutor.execute to task store operations
- Add GET /api/agents/:id/runs/:runId/mutations endpoint to query mutations by runId
- Add createTaskLogToolWithContext for heartbeat tools with run context support
- Add comprehensive tests for RunMutationContext across store and heartbeat modules
- Update memory.md with RunMutationContext usage convention

.fusion/memory.md

@@ -30,6 +30,7 @@
 - For conditionally rendered mobile inputs in dashboard components, prefer React `autoFocus` on the input over effect+`setTimeout` focus logic keyed to open-state booleans; mount timing is more reliable and simpler.
 - Checkout leasing is explicit: use `checkoutTask`/`releaseTask` (or `/api/tasks/:id/checkout` + `/release`) for ownership, treat 409 conflicts as non-retryable contention, and let `HeartbeatMonitor.executeHeartbeat()` only validate `checkedOutBy` (never auto-acquire leases).
 - The null-as-delete pattern for settings: In `TaskStore.updateSettings()`, `null` values in the settings patch are treated as "delete this key from settings" (since `JSON.stringify` drops `undefined` keys). This allows the frontend to explicitly clear a setting by sending `null`. The key is deleted from both `config.settings` and `projectPatch` before merging, so cleared settings fall back to `DEFAULT_SETTINGS`.
+- `TaskStore.logEntry()`, `addComment()`, `addSteeringComment()`, `pauseTask()` accept an optional `RunMutationContext` parameter for audit trail correlation. Always pass it when the caller is an engine module (executor, heartbeat monitor) to maintain the audit trail. The executor constructs a synthetic `runContext` with `runId: "exec-{taskId}-{timestamp}-{random}"` since it doesn't use `AgentHeartbeatRun`.
 
 ## Color Theme System