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_issues.go

@@ -35,6 +35,13 @@ Examples:
   # Move issues with label filter
   bd migrate-issues --from . --to ~/feature-work --label frontend --label urgent`,
 	Run: func(cmd *cobra.Command, args []string) {
+		dryRun, _ := cmd.Flags().GetBool("dry-run")
+
+		// Block writes in readonly mode
+		if !dryRun {
+			CheckReadonly("migrate-issues")
+		}
+
 		ctx := rootCtx
 
 		// Parse flags
@@ -48,7 +55,6 @@ Examples:
 		idsFile, _ := cmd.Flags().GetString("ids-file")
 		include, _ := cmd.Flags().GetString("include")
 		withinFromOnly, _ := cmd.Flags().GetBool("within-from-only")
-		dryRun, _ := cmd.Flags().GetBool("dry-run")
 		strict, _ := cmd.Flags().GetBool("strict")
 		yes, _ := cmd.Flags().GetBool("yes")