codeSplitter line locks 1: leases everywhere #926

Design: apps/codesplitter/docs/line-locks.md (#925).

• Put claims in openapi.ts (list_claims, claim_lines, release_claim, claim on edit_lines), so MCP, the CLI and the docs site get them. Add palette commands too, so WebMCP gets them
• A 409 carries claims (who, task, until) and retryAfter
• Agent leases default to 2 min, and any edit carrying claim renews it. Add an explicit renew and a total-age cap (about 30 min unless a person extends it)
• Expiry by DO alarm at the earliest until, re-armed in the constructor (desktop and box alarms are setTimeouts and are lost on restart), with an unclaim broadcast
• A heartbeat in agent-run.ts while the CLI thinks. Held conflicts tell the agent to work elsewhere
• Tests on the local twin: crash → lapse → unclaim, evict() mid-lease, re-arm after restart