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/autoimport.go

@@ -12,6 +12,7 @@ import (
 	"runtime"
 	"strings"
 
+	"github.com/steveyegge/beads/internal/git"
 	"github.com/steveyegge/beads/internal/storage"
 	"github.com/steveyegge/beads/internal/syncbranch"
 	"github.com/steveyegge/beads/internal/types"
@@ -199,6 +200,22 @@ func findBeadsDir() string {
 		return ""
 	}
 
+	// Check if we're in a git worktree with sparse checkout
+	// In worktrees, .beads might not exist locally, so we need to resolve to the main repo
+	if git.IsWorktree() {
+		mainRepoRoot, err := git.GetMainRepoRoot()
+		if err == nil && mainRepoRoot != "" {
+			mainBeadsDir := filepath.Join(mainRepoRoot, ".beads")
+			if info, err := os.Stat(mainBeadsDir); err == nil && info.IsDir() {
+				resolved, err := filepath.EvalSymlinks(mainBeadsDir)
+				if err != nil {
+					return mainBeadsDir
+				}
+				return resolved
+			}
+		}
+	}
+
 	for {
 		beadsDir := filepath.Join(dir, ".beads")
 		if info, err := os.Stat(beadsDir); err == nil && info.IsDir() {