feat: file-read decision gate — block reads when observation history exists

Add a PreToolUse gate that blocks file reads on first attempt when rich
observation history exists, presenting the timeline as feedback. Claude
then decides: use get_observations() (skip read, save tokens) or re-read
(allowed on second attempt).

- FileReadGate: in-memory session-scoped gate with 4h TTL
- POST /api/file-context/gate endpoint in worker
- stderrMessage plumbing in hook-command for exit code 2
- file-context handler uses gate to block/allow reads

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/worker/http/routes/DataRoutes.ts

@@ -19,6 +19,7 @@ import { SSEBroadcaster } from '../../SSEBroadcaster.js';
 import type { WorkerService } from '../../../worker-service.js';
 import { BaseRouteHandler } from '../BaseRouteHandler.js';
 import { getObservationsByFilePath } from '../../../sqlite/observations/get.js';
+import { checkAndMark } from '../../FileReadGate.js';
 
 export class DataRoutes extends BaseRouteHandler {
   constructor(
@@ -62,6 +63,9 @@ export class DataRoutes extends BaseRouteHandler {
 
     // Import endpoint
     app.post('/api/import', this.handleImport.bind(this));
+
+    // File-context gate
+    app.post('/api/file-context/gate', this.handleFileContextGate.bind(this));
   }
 
   /**
@@ -496,4 +500,20 @@ export class DataRoutes extends BaseRouteHandler {
       clearedCount
     });
   });
+
+  /**
+   * Check if a file has already had its timeline injected in this session
+   * POST /api/file-context/gate
+   * Body: { sessionId: string, filePath: string }
+   * Returns: { firstAttempt: boolean }
+   */
+  private handleFileContextGate = this.wrapHandler((req: Request, res: Response): void => {
+    const { sessionId, filePath } = req.body;
+    if (!sessionId || !filePath) {
+      this.badRequest(res, 'sessionId and filePath are required');
+      return;
+    }
+    const firstAttempt = checkAndMark(sessionId, filePath);
+    res.json({ firstAttempt });
+  });
 }