feat: add Git worktree compatibility (PR #478)

Adds comprehensive Git worktree support for beads issue tracking:

Core changes:
- New internal/git/gitdir.go package for worktree detection
- GetGitDir() returns proper .git location (main repo, not worktree)
- Updated all hooks to use git.GetGitDir() instead of local helper
- BeadsDir() now prioritizes main repository's .beads directory

Features:
- Hooks auto-install in main repo when run from worktree
- Shared .beads directory across all worktrees
- Config option no-install-hooks to disable auto-install
- New bd worktree subcommand for diagnostics

Documentation:
- New docs/WORKTREES.md with setup instructions
- Updated CHANGELOG.md and AGENT_INSTRUCTIONS.md

Testing:
- Updated tests to use exported git.GetGitDir()
- Added worktree detection tests

Co-authored-by: Claude <noreply@anthropic.com>
Closes: #478

cmd/bd/templates/hooks/pre-commit

@@ -53,9 +53,19 @@ if ! bd sync --flush-only >/dev/null 2>&1; then
 fi
 
 # Stage all tracked JSONL files (beads.jsonl, issues.jsonl for backward compat, deletions.jsonl for deletion propagation)
-# git add is harmless if file doesn't exist
-for f in .beads/beads.jsonl .beads/issues.jsonl .beads/deletions.jsonl; do
-    [ -f "$f" ] && git add "$f" 2>/dev/null || true
-done
+# For worktrees, .beads is in the main repo's working tree, not the worktree,
+# so we can't use git add. Skip staging for worktrees.
+if [ "$(git rev-parse --git-dir)" = "$(git rev-parse --git-common-dir)" ]; then
+    # Regular repo: files are in the working tree, safe to add
+    # git add is harmless if file doesn't exist
+    for f in .beads/beads.jsonl .beads/issues.jsonl .beads/deletions.jsonl; do
+        [ -f "$f" ] && git add "$f" 2>/dev/null || true
+    done
+else
+    # Worktree: .beads is in the main repo's working tree, not this worktree
+    # Git rejects adding files outside the worktree, so we skip it.
+    # The main repo will see the changes on the next pull/sync.
+    :  # do nothing
+fi
 
 exit 0