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/merge/merge_test.go

@@ -1648,7 +1648,7 @@ func TestMerge3WayWithTTL(t *testing.T) {
 		left := []Issue{tombstone}
 		right := []Issue{liveIssue}
 
-		result, _ := merge3WayWithTTL(base, left, right, shortTTL)
+		result, _ := Merge3WayWithTTL(base, left, right, shortTTL)
 		if len(result) != 1 {
 			t.Fatalf("expected 1 issue, got %d", len(result))
 		}
@@ -1665,7 +1665,7 @@ func TestMerge3WayWithTTL(t *testing.T) {
 		left := []Issue{tombstone}
 		right := []Issue{liveIssue}
 
-		result, _ := merge3WayWithTTL(base, left, right, longTTL)
+		result, _ := Merge3WayWithTTL(base, left, right, longTTL)
 		if len(result) != 1 {
 			t.Fatalf("expected 1 issue, got %d", len(result))
 		}