feat: Implement no-tmux mode with beads as data plane (gt-vc3l4)

Enable Gas Town to operate without tmux by using beads for args transport:

- Add `attached_args` field to beads AttachmentFields
- gt sling: Store args in bead description, graceful fallback if no tmux
- gt prime: Display attached args prominently on startup
- gt mol status: Include attached_args in status output
- gt spawn --naked: Assign work via mail only, skip tmux session

Agents discover args via gt prime / bd show when starting manually.
Docs added explaining what works vs degraded behavior in no-tmux mode.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/prime.go

@@ -572,6 +572,11 @@ func outputAttachmentStatus(ctx RoleContext) {
 	if attachment.AttachedAt != "" {
 		fmt.Printf("Attached at: %s\n", attachment.AttachedAt)
 	}
+	if attachment.AttachedArgs != "" {
+		fmt.Println()
+		fmt.Printf("%s\n", style.Bold.Render("📋 ARGS (use these to guide execution):"))
+		fmt.Printf("  %s\n", attachment.AttachedArgs)
+	}
 	fmt.Println()
 
 	// Show current step from molecule