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/formula/controlflow.go

@@ -212,6 +212,8 @@ func expandLoopWithVars(step *Step, vars map[string]string) ([]*Step, error) {
 // expandLoopIteration expands a single iteration of a loop.
 // The iteration index is used to generate unique step IDs.
 // The iterVars map contains loop variable bindings for this iteration (gt-8tmz.27).
+//
+//nolint:unparam // error return kept for API consistency with future error handling
 func expandLoopIteration(step *Step, iteration int, iterVars map[string]string) ([]*Step, error) {
 	result := make([]*Step, 0, len(step.Loop.Body))