#11878: fix: add missing error handler to execDocker child process

## Summary - `execDocker()` in `src/agents/sandbox/docker.ts` only attaches a `close` event handler to the spawned child process - If `docker` is not installed or cannot be executed, `spawn` emits an `error` event (ENOENT) that goes unhandled — the returned Promise hangs indefinitely and Node.js may crash with an unhandled exception - Adds `child.on("error", reject)` to match the pattern used by other spawn callsites in the project (`bash-tools.exec.ts:767`, `runner.ts:474`, `entry.ts:67`, etc.) ## Test plan - [x] Existing sandbox tests pass - [x] One-line change, follows established project pattern Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com> <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates `execDocker()` (`src/agents/sandbox/docker.ts`) to attach an `error` event handler to the spawned `docker` child process (`child.on("error", reject)`). This ensures the returned Promise rejects (instead of hanging / crashing on an unhandled error event) when `spawn("docker", ...)` fails immediately (e.g., `ENOENT` when Docker is not installed or executable). The change aligns `execDocker()` with other spawn callsites in the repo that already handle `error` events. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is a single, targeted addition that fixes a definite failure mode (unhandled child process `error` events) without altering normal-success behavior. It matches established patterns for `spawn()` usage and narrows the chance of hung Promises or process crashes when Docker is missing/unexecutable. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->