muster v0.2.3 — generic setup (contrib/) + README rewrite
Everything a cloner needs now ships in the repo: contrib/ carries a generic session hook script (auto-register + self-resolving inbox), copy-paste hook configs for Claude Code and Codex, and the two-line tmux mailbox render. README rewritten around the generic setup path — any MCP client can join; per-project tmux servers and hooks are optional layers. Also fixes stale docs (Codex nudge now auto-submits). No Go changes.