feat: add runner log capture and debug endpoint

- Drop exec from run.sh so tee can capture output to /tmp/runner.log
- Add POST /debug-runner-log endpoint to read logs from running sandboxes
  by job_id, enabling external debugging while sandbox is alive

Author: manascb1344

app.py

@@ -747,7 +747,7 @@ async def github_webhook(request: Request):
             " && cd /actions-runner"
             " && export RUNNER_ALLOW_RUNASROOT=1"
             " && export DOCKER_BUILDKIT=1"
-            " && exec ./run.sh --jitconfig $GHA_JIT_CONFIG"
+            " && ./run.sh --jitconfig $GHA_JIT_CONFIG 2>&1 | tee /tmp/runner.log"
         )
 
         sandbox_kwargs = dict(
@@ -835,3 +835,27 @@ async def github_webhook(request: Request):
                     "limit": MAX_CONCURRENT_PER_REPO,
                 },
             )
+
+
+@app.function(image=runner_image, secrets=[github_secret])
+@modal.fastapi_endpoint(method="POST")
+async def debug_runner_log(request: Request):
+    """Read /tmp/runner.log from a running sandbox by job_id."""
+    try:
+        body = await request.json()
+        job_id = body.get("job_id")
+        if not job_id:
+            raise HTTPException(status_code=400, detail="Missing job_id")
+
+        for sb in modal.Sandbox.list(app_id=app.app_id, tags={"job_id": str(job_id)}):
+            try:
+                log = sb.filesystem.read_text("/tmp/runner.log")
+                return {"job_id": job_id, "log": log}
+            except Exception:
+                continue
+
+        return {"job_id": job_id, "log": "Sandbox not found or log not available"}
+    except HTTPException:
+        raise
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=f"Error reading runner log: {e}")