Replace manual in-process _SESSION_LEASE seeding with an auditable
gitea_adopt_merger_pr_lease tool that posts durable adoption proof and
records sanctioned session provenance. Mutation gates now reject bare
record_session_lease calls without provenance from acquire/adopt/heartbeat.
Closes#536
Structured PR-thread leases with acquire/heartbeat MCP tools and
mutation-time enforcement so reviewers cannot race the same PR queue.
Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>