Archive a-lease-says-who

[VeryComplexAndLongName]

Closes the change. Every task is ticked, including 5.4, which another agent closed with live evidence in #413 — the lease a real chain run wrote, both command outputs, and all three exit codes.

Three requirements merge into openspec/specs/ci-cli:

• A lease records who took it, as attribution. The git identity of the working directory, reported as what it is — a self-declared label anybody can set, the same one that signs the repository's commits. Nothing is permitted or refused on the strength of it, and the message says "git author", never "user".
• The holder of a workspace can be asked about. Until now the only way to learn who held one was to try to start a run and read the refusal, which answers the question only at the moment you are being told no.
• A lease is cleared only where its holder is shown to be gone. A dead holder already heals: it stops renewing and the next acquirer reclaims the lease once the heartbeat is stale. A live one has the workspace open, and taking its lease would let a second mutating run start against files it still holds — which is what the lease exists to prevent. A stuck holder is stopped, not robbed.

openspec validate --all passes over 16 items.

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