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/templates/hooks/pre-commit

@@ -1,6 +1,6 @@
 #!/bin/sh
 # bd-shim v1
-# bd-hooks-version: 0.36.0
+# bd-hooks-version: 0.38.0
 #
 # bd (beads) pre-commit hook - thin shim
 #