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.

plugin/hooks/hooks.json

@@ -0,0 +1,14 @@
+{
+  "hooks": {
+    "UserPromptSubmit": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "claude-mailbox check --hook"
+          }
+        ]
+      }
+    ]
+  }
+}