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/README.md

@@ -1,8 +1,8 @@
 # claude-mailbox plugin
 
-Lets Claude Code pull unread messages from a local `claude-mailbox` daemon before every prompt and inject them into the conversation context.
+Lets Claude Code pull unread messages from a local `claude-mailbox` daemon before every prompt and inject them into the conversation context. Each Claude session gets a **unique mailbox identity** auto-derived from its session id, so two sessions in the same project never collide.
 
-## Setup (two steps, all inside Claude Code)
+## Setup (three prompts, all inside Claude Code)
 
 ```
 /plugin marketplace add https://git.kuns.dev/releases/ClaudeMailbox
@@ -10,50 +10,59 @@ Lets Claude Code pull unread messages from a local `claude-mailbox` daemon befor
 /claude-mailbox:mailbox-doctor
 ```
 
-The doctor command walks the rest:
+The doctor walks the rest:
 
-1. checks whether the `claude-mailbox` binary is on `PATH` — installs it (`npm install -g @kuns/claude-mailbox`) if missing, asks before doing anything that might need elevation
-2. checks the daemon status — runs `install-autostart` and/or `start` until it reports `Running`
-3. ensures `CLAUDE_MAILBOX_NAME` is set in `.claude/settings.json` env — prompts for a name if not, writes it idempotently
-4. runs a self → self smoke test to verify the round-trip works
+1. installs the `claude-mailbox` binary via `npm install -g @kuns/claude-mailbox` if missing (asks first)
+2. registers the daemon for autostart and starts it if needed
+3. health-probes `http://127.0.0.1:47822/health`
+4. optionally lets you set a **base prefix** (e.g., `backend`) — without one, mailbox names are anonymous (`claude-XXXXXXXX`)
+5. runs a self → self smoke test
 
-Restart Claude Code after the doctor finishes (only needed if the mailbox name was newly written). Unread messages will then appear in context before every prompt.
+Restart Claude Code only if step 4 wrote a new prefix. After that, every prompt auto-pulls unread messages.
 
-## Why a mailbox name?
+## Mailbox identity (the important bit)
 
-Each Claude session has an identity used to address peer sessions — like an email address. If you run a `backend` session and a `frontend` session in parallel, they need different names so they can send messages to each other.
+Each Claude Code session gets its own mailbox name, derived from the session's UUID:
 
-For a single Claude Code instance just wanting notifications, any stable kebab-case name works. The name lives in **per-project** `.claude/settings.json` env, so different worktrees / projects automatically get different mailboxes.
+| Configuration | Resulting mailbox name |
+|---|---|
+| No `CLAUDE_MAILBOX_NAME` set | `claude-a8b3c1d2` (first 8 hex chars of session_id) |
+| `CLAUDE_MAILBOX_NAME=backend` in `.claude/settings.json` env | `backend-a8b3c1d2` |
 
-## What the hook actually does
+So if you open two Claude Code sessions in the same project, they'll be e.g. `backend-a8b3c1d2` and `backend-d4e5f6a7` — distinct, addressable, no manual setup.
 
-Before every prompt the plugin runs `claude-mailbox check --hook`, which:
+The `SessionStart` hook announces the current session's mailbox name in the conversation context on startup. Peers discover each other via `claude-mailbox list` or the `mcp__mailbox__list_mailboxes` MCP tool.
 
-- 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 a missing daemon is loud, not invisible.
+## What the hooks do
 
-Cost: one local HTTP round-trip plus Node coldstart per prompt (~100ms on Windows).
+| Hook | Command | Effect |
+|---|---|---|
+| `SessionStart` | `claude-mailbox session-announce` | Prints `"Claude-Mailbox: this session is mailbox \`X\`"` so Claude knows its own identity. |
+| `UserPromptSubmit` | `claude-mailbox check --hook` | Pulls unread messages for the session's mailbox and injects them as context. Silent on empty inbox; emits a one-line setup hint when the daemon is unreachable. |
 
-## Commands
+Cost: one local HTTP round-trip per prompt + Node coldstart (~100ms on Windows).
+
+## Slash commands
 
 | Command | What it does |
 |---|---|
 | `/claude-mailbox:mailbox-doctor` | Diagnose + auto-fix the full setup. |
 | `/claude-mailbox:mailbox-status` | Read-only health check. No changes. |
+| `/claude-mailbox:mailbox-update` | Update the daemon to the latest npm version and restart it. |
 
-## Smoke test (manually, after doctor finishes)
+## Sending a message to a peer session
+
+From inside Claude Code, use the MCP tool (the daemon already exposes `mcp__mailbox__*`). From any shell:
 
 ```sh
-claude-mailbox send --from probe --to <your-mailbox-name> --body "hello"
+claude-mailbox list                                  # find the recipient's mailbox name
+claude-mailbox send --from probe --to backend-a8b3c1d2 --body "hi"
 ```
 
-Then start a new Claude Code prompt — 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
+claude-mailbox uninstall-autostart    # if you registered it
 ```