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>

docs/ARCHITECTURE.md

@@ -275,7 +275,7 @@ open ──▶ in_progress ──▶ closed
 
 ```
 ┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
-│ bd wisp create  │───▶│  Wisp Issues    │───▶│  bd mol squash  │
+│   bd mol wisp       │───▶│  Wisp Issues    │───▶│  bd mol squash  │
 │ (from template) │    │  (local-only)   │    │  (→ digest)     │
 └─────────────────┘    └─────────────────┘    └─────────────────┘
 ```