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/show.go

@@ -456,6 +456,7 @@ var updateCmd = &cobra.Command{
 	Short: "Update one or more issues",
 	Args:  cobra.MinimumNArgs(1),
 	Run: func(cmd *cobra.Command, args []string) {
+		CheckReadonly("update")
 		jsonOutput, _ := cmd.Flags().GetBool("json")
 		updates := make(map[string]interface{})
 
@@ -717,6 +718,7 @@ Examples:
   bd edit bd-42 --acceptance       # Edit acceptance criteria`,
 	Args: cobra.ExactArgs(1),
 	Run: func(cmd *cobra.Command, args []string) {
+		CheckReadonly("edit")
 		id := args[0]
 		ctx := rootCtx
 
@@ -904,6 +906,7 @@ var closeCmd = &cobra.Command{
 	Short: "Close one or more issues",
 	Args:  cobra.MinimumNArgs(1),
 	Run: func(cmd *cobra.Command, args []string) {
+		CheckReadonly("close")
 		reason, _ := cmd.Flags().GetString("reason")
 		if reason == "" {
 			reason = "Closed"