refactor: remove unused bd pin/unpin/hook commands (bd-x0zl)

Analysis found these commands are dead code:
- gt never calls `bd pin` - uses `bd update --status=pinned` instead
- Beads.Pin() wrapper exists but is never called
- bd hook functionality duplicated by gt mol status
- Code comment says "pinned field is cosmetic for bd hook visibility"

Removed:
- cmd/bd/pin.go
- cmd/bd/unpin.go
- cmd/bd/hook.go

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/doctor/maintenance.go

@@ -312,7 +312,7 @@ func CheckCompactionCandidates(path string) DoctorCheck {
 // the actual beads directory location.
 func resolveBeadsDir(beadsDir string) string {
 	redirectFile := filepath.Join(beadsDir, "redirect")
-	data, err := os.ReadFile(redirectFile)
+	data, err := os.ReadFile(redirectFile) //nolint:gosec // redirect file path is constructed from known beadsDir
 	if err != nil {
 		// No redirect file - use original path
 		return beadsDir