feat: multi-repo workspace support with auto-detection and project root resolution

Add support for workspaces with multiple independent git repositories.
When configured, GSD routes commits to the correct sub-repo and ensures
.planning/ stays at the project root.

Core features:
- detectSubRepos(): scans child directories for .git to discover repos
- findProjectRoot(): walks up from CWD to find the project root that
  owns .planning/, preventing orphaned .planning/ in sub-repos
- loadConfig auto-syncs sub_repos when repos are added or removed
- Migrates legacy "multiRepo: true" to sub_repos array automatically
- All init commands include project_root in output
- cmdCommitToSubrepo: groups files by sub-repo prefix, commits independently

Zero impact on single-repo workflows — sub_repos defaults to empty array.

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

agents/gsd-executor.md

@@ -47,7 +47,7 @@ INIT=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" init execute-phase "
 if [[ "$INIT" == @file:* ]]; then INIT=$(cat "${INIT#@file:}"); fi
 ```
 
-Extract from init JSON: `executor_model`, `commit_docs`, `phase_dir`, `plans`, `incomplete_plans`.
+Extract from init JSON: `executor_model`, `commit_docs`, `sub_repos`, `phase_dir`, `plans`, `incomplete_plans`.
 
 Also read STATE.md for position, decisions, blockers:
 ```bash
@@ -328,6 +328,14 @@ git add src/types/user.ts
 | `chore`    | Config, tooling, dependencies                   |
 
 **4. Commit:**
+
+**If `sub_repos` is configured (non-empty array from init context):** Use `commit-to-subrepo` to route files to their correct sub-repo:
+```bash
+node ~/.claude/get-shit-done/bin/gsd-tools.cjs commit-to-subrepo "{type}({phase}-{plan}): {concise task description}" --files file1 file2 ...
+```
+Returns JSON with per-repo commit hashes: `{ committed: true, repos: { "backend": { hash: "abc", files: [...] }, ... } }`. Record all hashes for SUMMARY.
+
+**Otherwise (standard single-repo):**
 ```bash
 git commit -m "{type}({phase}-{plan}): {concise task description}