feat(FN-1877): add auto-compaction on context window errors to promptWithFallback

- Detect context-window-limit errors from AI sessions and automatically compact
  the session conversation history before retrying (one attempt per session)
- Centralize auto-compaction in promptWithFallback (pi.ts) so executor, merger,
  and step-session-executor all benefit from the same mechanism
- Remove scattered context-limit error handling from executor.ts, merger.ts, and
  step-session-executor.ts in favor of the centralized approach
- Add comprehensive tests for auto-compaction retry behavior in pi.test.ts
- Remove unused compactSessionContext import from step-session-executor.ts
- Add memory note documenting the centralized auto-compaction design

.fusion/memory.md

@@ -3,6 +3,7 @@
 ## Architecture
 
 - `TaskExecutor` terminates active agent sessions (single and step) when tasks are moved away from `in-progress` via the `task:moved` event handler. This prevents zombie sessions when users manually send tasks back to todo/triage from the board UI.
+- **Centralized Context-Window Auto-Compaction (FN-1877)**: The `promptWithFallback()` function in `packages/engine/src/pi.ts` automatically catches context-window overflow errors, runs `compactSessionContext()`, and retries once. This centralizes recovery for ALL agent types (executor, step-session, merger, triage, heartbeat, reviewer, mission-execution-loop). Callers that previously had duplicate compact-and-resume logic (executor, step-session-executor, merger) have been simplified to use `promptWithFallback`'s auto-compaction as first-level recovery, with their own reduced-prompt fallbacks as second-level recovery. This eliminates code duplication and ensures consistent recovery behavior.
 - **Workflow Step Revision Loop (FN-1499)**: Workflow steps can request implementation revisions via "REQUEST REVISION" output. The flow:
   1. Workflow step agent outputs "REQUEST REVISION\n\n[feedback]" to signal that code changes are needed
   2. `executeWorkflowStep()` detects this pattern and returns `WorkflowStepOutcome` with `revisionRequested: true`