chore: add murmur-handoff skill

[wine-fall]

Summary

Adds a murmur-handoff skill that emits one paste-ready prompt to continue
murmur work in a fresh session (which holds none of the current thread's
context).

• With args — captures the handoff the user states, filling gaps from
  session context.
• No args — infers the next step from live signals (git status/log,
  specs/STATUS.md, any open PR + CI, the thread itself) and hands that off.

The emitted prompt is self-contained: it orients the receiver to the repo/branch
and routing card, states committed-vs-dirty work, names the one next task
(routing spec builds through murmur-build-spec/murmur-ship), flags settled
decisions and real-boundary caveats, and gives the acceptance check.

AI coding brief

• Original request: "加一个 murmur-handoff 的 skill" — summarize a session's
  handoff requirements into a prompt that pastes straight into another session
  and runs. When invoked without a prompt, default to inferring the next step
  from the current session. The why: make cross-session handoff a one-command,
  copy-paste operation instead of hand-writing context each time.
• Manual interventions: none on the skill content. The user then directed
  the delivery: open it from a fresh worktree (the main checkout is shared),
  raise a PR, and merge once CI is green.
• Retro: the request was crisp and needed no clarification — both modes and
  the default behavior were specified up front. Stating the delivery path
  (worktree → PR → merge-on-green) in the same turn kept it a single pass.

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