feat: add --readonly flag for worker sandboxes (gt-ymo)

Add a --readonly flag that blocks all write operations, allowing workers
to read beads state without modifying it. Workers can use:
- bd show, bd list, bd ready (read operations)

Workers cannot use:
- bd create, bd update, bd close, bd sync, etc. (write operations)

The flag can be set via:
- --readonly flag on command line
- BD_READONLY=true environment variable
- readonly: true in config file

This enables swarm workers to see their assigned work from a static
snapshot of the beads database without accidentally modifying it.

Commands protected by readonly mode:
- create, update, close, delete, edit
- sync, import, reopen
- comment add, dep add/remove, label add/remove
- repair-deps, compact, migrate, migrate-hash-ids, migrate-issues
- rename-prefix, validate --fix-all, duplicates --auto-merge
- epic close-eligible, jira sync

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/migrate_hash_ids.go

@@ -48,9 +48,14 @@ EXAMPLES:
 WARNING: Backup your database before running this command, even though it creates one automatically.`,
 	Run: func(cmd *cobra.Command, _ []string) {
 		dryRun, _ := cmd.Flags().GetBool("dry-run")
-		
+
+		// Block writes in readonly mode
+		if !dryRun {
+			CheckReadonly("migrate-hash-ids")
+		}
+
 		ctx := rootCtx
-		
+
 		// Find database
 		dbPath := beads.FindDatabasePath()
 		if dbPath == "" {