feat(plugin): ship Claude Code plugin + marketplace manifest

Adds a /plugin marketplace at the repo root and a `claude-mailbox` plugin under
plugin/ that wires the UserPromptSubmit hook without needing the per-user
`install-hook` step. The hook command (`claude-mailbox check --hook`) now reads
the mailbox name from $CLAUDE_MAILBOX_NAME when --name is omitted and emits a
one-line setup hint when the daemon is unreachable, so a missing daemon is loud
instead of invisible.

The plugin only contains the Claude Code glue — the daemon binary is still a
separate prerequisite (`npm i -g @kuns/claude-mailbox` + install-autostart),
and the plugin/README plus main README spell out the three-step setup.

.claude-plugin/marketplace.json

@@ -0,0 +1,14 @@
+{
+  "name": "claude-mailbox",
+  "owner": {
+    "name": "Mika Kuns"
+  },
+  "description": "Plugins for the Claude-Mailbox project.",
+  "plugins": [
+    {
+      "name": "claude-mailbox",
+      "source": "./plugin",
+      "description": "Auto-checks the local Claude-Mailbox daemon before every prompt and injects pending messages."
+    }
+  ]
+}