feat(naming)!: auto-derive mailbox name from project + runtime rename

Mailbox names are now built as <project>-<session-short>, where <project>
is the sanitized git-repo basename (or cwd basename) — no more env-var
prefix step. Sessions can re-tag themselves at runtime via the new
mcp__mailbox__rename tool (POST /v1/rename), which transfers all
pending messages to the new name in a single transaction. Peers using
the old name re-discover via list_mailboxes.

BREAKING: \$CLAUDE_MAILBOX_NAME is no longer read. Existing setups that
relied on the env-var prefix should remove it from .claude/settings.json;
the prefix now comes from the working directory automatically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/commands/mailbox-status.md

@@ -12,12 +12,12 @@ Claude-Mailbox status
   binary:       <output of `claude-mailbox --version`, or "not installed">
   daemon:       <output of `claude-mailbox status`>
   health:       <"ok" if GET http://127.0.0.1:37849/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">
+  mailbox name: auto-derived per session as <project>-<short-session-id> (see SessionStart announcement)
+  pending:      n/a (the session's mailbox name isn't known until SessionStart runs in this session's context)
 ```
 
 End with one line:
 
 - All good → `Status: OK`
-- Missing daemon or unset name → `Status: Setup incomplete. Run /claude-mailbox:mailbox-doctor to fix.`
+- Missing daemon → `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.`