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/README.md

@@ -0,0 +1,68 @@
+# claude-mailbox plugin
+
+Lets Claude Code automatically pull unread messages from a local `claude-mailbox` daemon before every prompt and inject them into the conversation context.
+
+## Three-step setup
+
+The Claude Code plugin itself is just the glue — you still need the daemon binary on PATH and a mailbox name.
+
+### 1. Install the daemon (one-time, per machine)
+
+```sh
+npm config set @kuns:registry=https://git.kuns.dev/api/packages/releases/npm/
+npm install -g @kuns/claude-mailbox
+claude-mailbox install-autostart   # registers per-OS autostart (no admin needed by default)
+```
+
+Verify with:
+
+```sh
+claude-mailbox status     # expect: Running
+```
+
+### 2. Install the plugin
+
+In Claude Code:
+
+```
+/plugin marketplace add https://git.kuns.dev/releases/ClaudeMailbox
+/plugin install claude-mailbox@claude-mailbox
+```
+
+### 3. Choose a mailbox name
+
+Set the env var Claude Code will inherit (any unique name — your username, machine name, whatever):
+
+```sh
+# Windows (PowerShell, persistent)
+setx CLAUDE_MAILBOX_NAME alice
+
+# macOS / Linux (add to ~/.zshrc or ~/.bashrc)
+export CLAUDE_MAILBOX_NAME=alice
+```
+
+Restart Claude Code so the new env var is picked up.
+
+## What happens at runtime
+
+Before every prompt the plugin runs `claude-mailbox check --hook`, which:
+
+- prints unread mailbox messages in a Claude-friendly format and marks them delivered,
+- stays **silent** when the inbox is empty or `CLAUDE_MAILBOX_NAME` is not set,
+- emits a one-line setup hint when the daemon is unreachable (so missing step 1 is loud).
+
+## Sending yourself a message (smoke test)
+
+```sh
+claude-mailbox send --from probe --to alice --body "hello from the CLI"
+```
+
+Then start a new Claude Code session — the message should appear in context before Claude's first reply.
+
+## Uninstall
+
+```
+/plugin uninstall claude-mailbox@claude-mailbox
+npm uninstall -g @kuns/claude-mailbox
+claude-mailbox uninstall-autostart   # if you used it
+```