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

go.mod

@@ -5,16 +5,17 @@ go 1.24.0
 toolchain go1.24.2
 
 require (
-	github.com/anthropics/anthropic-sdk-go v1.19.0
+	github.com/anthropics/anthropic-sdk-go v1.18.1
 	github.com/fatih/color v1.18.0
 	github.com/fsnotify/fsnotify v1.9.0
-	github.com/ncruces/go-sqlite3 v0.30.3
-	github.com/spf13/cobra v1.10.2
+	github.com/google/go-cmp v0.7.0
+	github.com/ncruces/go-sqlite3 v0.30.1
+	github.com/spf13/cobra v1.10.1
 	github.com/spf13/viper v1.21.0
 	github.com/tetratelabs/wazero v1.10.1
-	golang.org/x/mod v0.31.0
-	golang.org/x/sys v0.39.0
-	golang.org/x/term v0.38.0
+	golang.org/x/mod v0.30.0
+	golang.org/x/sys v0.38.0
+	golang.org/x/term v0.37.0
 	gopkg.in/natefinch/lumberjack.v2 v2.2.1
 	gopkg.in/yaml.v3 v3.0.1
 	rsc.io/script v0.0.2
@@ -22,7 +23,6 @@ require (
 
 require (
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
-	github.com/google/go-cmp v0.7.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
@@ -39,6 +39,6 @@ require (
 	github.com/tidwall/pretty v1.2.1 // indirect
 	github.com/tidwall/sjson v1.2.5 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/text v0.31.0 // indirect
-	golang.org/x/tools v0.39.0 // indirect
+	golang.org/x/text v0.30.0 // indirect
+	golang.org/x/tools v0.38.0 // indirect
 )