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/doctor/fix/symlink_test.go

@@ -3,7 +3,6 @@ package fix
 import (
 	"os"
 	"path/filepath"
-	"runtime"
 	"testing"
 )
 
@@ -14,9 +13,6 @@ import (
 // - When .beads is a symlink, Permissions() should return nil without changing anything
 // - This prevents attempts to chmod symlink targets (which may be read-only like /nix/store)
 func TestPermissions_SkipsSymlinkedBeadsDir(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("Skipping symlink test on Windows - requires elevated privileges")
-	}
 	tmpDir := t.TempDir()
 
 	// Create target .beads directory with wrong permissions
@@ -66,9 +62,6 @@ func TestPermissions_SkipsSymlinkedBeadsDir(t *testing.T) {
 // TestPermissions_SkipsSymlinkedDatabase verifies that chmod is skipped for
 // symlinked database files, but .beads directory permissions are still fixed.
 func TestPermissions_SkipsSymlinkedDatabase(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("Skipping symlink test on Windows - requires elevated privileges")
-	}
 	tmpDir := t.TempDir()
 
 	// Create real .beads directory with wrong permissions
@@ -132,9 +125,6 @@ func TestPermissions_SkipsSymlinkedDatabase(t *testing.T) {
 // TestPermissions_FixesRegularFiles verifies that permissions ARE fixed for
 // regular (non-symlinked) files.
 func TestPermissions_FixesRegularFiles(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("Skipping permissions test on Windows - Unix-style permissions don't apply")
-	}
 	tmpDir := t.TempDir()
 
 	// Create .beads directory with wrong permissions