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

scripts/bump-version.sh

@@ -230,13 +230,7 @@ main() {
             "# bd-hooks-version: $NEW_VERSION"
     done
 
-    # 10. Update default.nix
-    echo "  • default.nix"
-    update_file "default.nix" \
-        "version = \"$CURRENT_VERSION\"" \
-        "version = \"$NEW_VERSION\""
-
-    # 11. Update CHANGELOG.md
+    # 10. Update CHANGELOG.md
     echo "  • CHANGELOG.md"
     update_changelog "$NEW_VERSION"
 
@@ -259,7 +253,6 @@ main() {
         "$(grep '__version__ = ' integrations/beads-mcp/src/beads_mcp/__init__.py | sed 's/.*"\(.*\)".*/\1/')"
         "$(jq -r '.version' npm-package/package.json)"
         "$(grep '# bd-hooks-version: ' cmd/bd/templates/hooks/pre-commit | sed 's/.*: \(.*\)/\1/')"
-        "$(grep 'version = ' default.nix | sed 's/.*"\(.*\)".*/\1/')"
     )
 
     ALL_MATCH=true
@@ -379,8 +372,7 @@ main() {
                 integrations/beads-mcp/src/beads_mcp/__init__.py \
                 npm-package/package.json \
                 README.md \
-                cmd/bd/templates/hooks/* \
-                default.nix
+                cmd/bd/templates/hooks/*
 
         # Add PLUGIN.md if it exists
         if [ -f "PLUGIN.md" ]; then
@@ -399,7 +391,6 @@ Updated all component versions:
 - Plugin: $CURRENT_VERSION → $NEW_VERSION
 - MCP server: $CURRENT_VERSION → $NEW_VERSION
 - npm package: $CURRENT_VERSION → $NEW_VERSION
-- Nix package: $CURRENT_VERSION → $NEW_VERSION
 - Documentation: $CURRENT_VERSION → $NEW_VERSION
 
 Generated by scripts/bump-version.sh"