fix(plugin): sync plugin.json version with releases

plugin.json was stuck at 0.1.0 across every release, so Claude Code's
per-version plugin cache never refreshed and clients kept running the
original .mcp.json (http://127.0.0.1:47822/mcp). Bump to 1.2.0 to match
the node package and add a release-workflow step that derives plugin.json
from the tag on every future release.

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": "0.1.0",
+  "version": "1.2.0",
   "description": "Auto-checks the local Claude-Mailbox daemon before every prompt and injects pending messages into the conversation context.",
   "author": {
     "name": "Mika Kuns"