chore(release): 1.5.4

Make /collaborate slash command self-contained so it works without a registered mailbox-collaborate skill in the session.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "claude-mailbox",
-  "version": "1.5.3",
+  "version": "1.5.4",
   "description": "Auto-checks the local Claude-Mailbox daemon before every prompt and after each subagent run, and injects pending messages into the conversation context.",
   "author": {
     "name": "Mika Kuns"