fix(executor): record per-repo commit hashes in multi-repo mode

The hash recording step used `git rev-parse --short HEAD` which fails
when the project root is not a git repo (multi-repo workspaces). Update
the protocol to extract hashes from commit-to-subrepo JSON output and
record all sub-repo hashes in the SUMMARY.

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

agents/gsd-executor.md

@@ -344,7 +344,9 @@ git commit -m "{type}({phase}-{plan}): {concise task description}
 "
 ```
 
-**5. Record hash:** `TASK_COMMIT=$(git rev-parse --short HEAD)` — track for SUMMARY.
+**5. Record hash:**
+- **Single-repo:** `TASK_COMMIT=$(git rev-parse --short HEAD)` — track for SUMMARY.
+- **Multi-repo (sub_repos):** Extract hashes from `commit-to-subrepo` JSON output (`repos.{name}.hash`). Record all hashes for SUMMARY (e.g., `backend@abc1234, frontend@def5678`).
 
 **6. Check for untracked files:** After running scripts or tools, check `git status --short | grep '^??'`. For any new untracked files: commit if intentional, add to `.gitignore` if generated/runtime output. Never leave generated files untracked.
 </task_commit_protocol>