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/mol_current.go

@@ -100,7 +100,7 @@ The output shows all steps with status indicators:
 				}
 				fmt.Println(".")
 				fmt.Println("\nTo start work on a molecule:")
-				fmt.Println("  bd pour <proto-id>          # Instantiate a molecule from template")
+				fmt.Println("  bd mol pour <proto-id>      # Instantiate a molecule from template")
 				fmt.Println("  bd update <step-id> --status in_progress  # Claim a step")
 				return
 			}