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/fix/hooks.go

@@ -28,7 +28,7 @@ func GitHooks(path string) error {
 	}
 
 	// Run bd hooks install
-	cmd := exec.Command(bdBinary, "hooks", "install") // #nosec G204 -- bdBinary from validated executable path
+	cmd := newBdCmd(bdBinary, "hooks", "install")
 	cmd.Dir = path                                     // Set working directory without changing process dir
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr