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/019_messaging_fields.go

@@ -20,10 +20,6 @@ func MigrateMessagingFields(db *sql.DB) error {
 	}{
 		{"sender", "TEXT DEFAULT ''"},
 		{"ephemeral", "INTEGER DEFAULT 0"},
-		{"replies_to", "TEXT DEFAULT ''"},
-		{"relates_to", "TEXT DEFAULT ''"},
-		{"duplicate_of", "TEXT DEFAULT ''"},
-		{"superseded_by", "TEXT DEFAULT ''"},
 	}
 
 	for _, col := range columns {
@@ -59,11 +55,5 @@ func MigrateMessagingFields(db *sql.DB) error {
 		return fmt.Errorf("failed to create sender index: %w", err)
 	}
 
-	// Add index for replies_to (for efficient thread queries)
-	_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_replies_to ON issues(replies_to) WHERE replies_to != ''`)
-	if err != nil {
-		return fmt.Errorf("failed to create replies_to index: %w", err)
-	}
-
 	return nil
 }