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>

internal/storage/sqlite/migrations/023_pinned_column.go

@@ -20,6 +20,11 @@ func MigratePinnedColumn(db *sql.DB) error {
 	}
 
 	if columnExists {
+		// Column exists (e.g. created by new schema); ensure index exists.
+		_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_pinned ON issues(pinned) WHERE pinned = 1`)
+		if err != nil {
+			return fmt.Errorf("failed to create pinned index: %w", err)
+		}
 		return nil
 	}