This implements the native merger lease acquisition tool to allow a merger session to acquire its own lease when a reviewer lease does not exist or has expired, unblocking #679 adoption.
Surface lease_proof_source / recovery reason from sanctioned session
provenance on gitea_merge_pr results, and block final reports that claim
manual/seeded/injected lease proof without MCP adoption evidence.
Closes#535
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
sysadmin