feat(plugin): per-session mailbox identity + mailbox-update command

The hook now derives a unique mailbox name from the session_id supplied
on hook stdin, so two parallel Claude Code sessions in the same project
get distinct mailboxes (e.g. `claude-a8b3c1d2`, `claude-d4e5f6a7`)
instead of colliding on a shared env value. An optional
CLAUDE_MAILBOX_NAME base prefix flavors the names as `<base>-<sid>`.

Adds:
- `claude-mailbox session-announce` subcommand for the new SessionStart
  hook, which prints the current session's mailbox name to context
- `/claude-mailbox:mailbox-update` slash command for `npm update` +
  daemon restart
- stdin parsing helpers (parseHookStdin, deriveSessionName) with unit
  tests; the doctor no longer needs a mandatory name prompt

plugin/commands/mailbox-update.md

@@ -0,0 +1,56 @@
+---
+description: Update the Claude-Mailbox daemon to the latest published npm version and restart it.
+allowed-tools: Bash
+---
+
+You are running the **Claude-Mailbox update** command. Update the `@kuns/claude-mailbox` npm package and restart the daemon. Never run `sudo` automatically — if elevation is needed, stop and ask.
+
+## Step 1 — current version
+
+Run: `claude-mailbox --version`
+
+- Exit 0 → record the version string as `CURRENT`.
+- Non-zero → tell the user the daemon binary is not installed yet. Suggest `/claude-mailbox:mailbox-doctor` to do a full setup and stop.
+
+## Step 2 — latest published version
+
+Run: `npm view @kuns/claude-mailbox version`
+
+If the npm registry config is missing, the call may fail with a 404. Fall back to:
+
+```
+npm view --registry=https://git.kuns.dev/api/packages/releases/npm/ @kuns/claude-mailbox version
+```
+
+Record the result as `LATEST`.
+
+## Step 3 — compare
+
+- If `CURRENT === LATEST`: print "Already up to date (vX.Y.Z)." and stop. Do not run any further steps.
+- Otherwise: tell the user `CURRENT` → `LATEST` and ask for confirmation before proceeding.
+
+## Step 4 — perform the update
+
+On user confirmation, run these in order. Stop on the first failure and report it:
+
+1. `claude-mailbox stop`
+2. `npm install -g @kuns/claude-mailbox@latest`
+   - On Linux/macOS this may fail with EACCES. **Do not run sudo automatically.** Ask the user how they want to proceed (e.g., `sudo npm install -g …`, or switch to a user-scoped Node setup with nvm/fnm).
+3. `claude-mailbox start`
+4. `claude-mailbox --version` to verify the upgrade landed.
+5. `claude-mailbox status` to verify the daemon is `Running`.
+
+## Step 5 — summary
+
+Print exactly this block:
+
+```
+Claude-Mailbox update
+  previous version: <CURRENT>
+  new version:      <whatever --version now reports>
+  daemon:           Running | Stopped | NotInstalled
+  pending messages survived: <count of messages still in inbox via `claude-mailbox list`, if applicable>
+```
+
+If the new version matches `LATEST` and daemon is `Running`, end with: "Update complete."
+Otherwise, end with the first thing that went wrong.