feat(plugin): add /claude-mailbox:mailbox-doctor for one-command setup

The doctor command runs entirely inside Claude Code and walks through:
binary install via npm, daemon autostart, mailbox-name prompt with write
to per-project `.claude/settings.json` env, and a self → self smoke test.
Also adds `/claude-mailbox:mailbox-status` as a read-only health check.

Reduces the colleague onboarding to: add marketplace, install plugin,
run the doctor — no terminal context-switch required.

plugin/commands/mailbox-status.md

@@ -0,0 +1,23 @@
+---
+description: Read-only Claude-Mailbox health check. No changes, no installs — just report.
+allowed-tools: Bash, Read
+---
+
+Report the Claude-Mailbox setup status without making any changes. If something is wrong, **tell** the user but **do not** fix it — suggest `/claude-mailbox:mailbox-doctor` for that.
+
+Print exactly this block, filling in each line:
+
+```
+Claude-Mailbox status
+  binary:       <output of `claude-mailbox --version`, or "not installed">
+  daemon:       <output of `claude-mailbox status`>
+  health:       <"ok" if GET http://127.0.0.1:47822/health returns 200, else "unreachable">
+  mailbox name: <value of env.CLAUDE_MAILBOX_NAME in ./.claude/settings.json, or "unset"; also note if ~/.claude/settings.json has a value>
+  pending:      <integer count from `claude-mailbox peek --name <resolved-name>` if name is set, else "n/a">
+```
+
+End with one line:
+
+- All good → `Status: OK`
+- Missing daemon or unset name → `Status: Setup incomplete. Run /claude-mailbox:mailbox-doctor to fix.`
+- Daemon installed but stopped → `Status: Daemon is not running. Try \`claude-mailbox start\` or run /claude-mailbox:mailbox-doctor.`