refactor(FN-2161): standardize on createFnAgent naming

- Rename engine export and call sites to use createFnAgent consistently across runtime flows
- Update core lazy engine loader and dashboard agent-generation/planning/chat paths to reference createFnAgent
- Refresh affected unit and integration tests, including renaming pi-create-kb-agent.test.ts to pi-create-fn-agent.test.ts
- Update AGENTS.md documentation references to match the new createFnAgent name

AGENTS.md

@@ -275,7 +275,7 @@ When debugging agent execution issues (agents stuck on "starting"), check these
 3. **`[executor] execute() called for FN-XXX`** — Confirms execute() was entered
 4. **`[executor] FN-XXX: worktree ready at ...`** — Confirms worktree creation
 5. **`[executor] FN-XXX: creating agent session`** — Confirms model resolution and session creation started
-6. **`[pi] createKbAgent called`** — Confirms the agent factory was invoked
+6. **`[pi] createFnAgent called`** — Confirms the agent factory was invoked
 7. **`[pi] Session created successfully`** — Confirms the AI session was created
 8. **`[executor] FN-XXX: calling promptWithFallback()...`** — Confirms the prompt was sent
 9. **`[stuck-detector] Tracking task FN-XXX`** — Confirms heartbeat monitoring started