gitea_acquire_merger_pr_lease previously continued when GET /pulls returned
an empty head SHA. Fail closed so exact-head scoping never proceeds with an
unresolvable live head. Add regression coverage for empty head payloads.
Fixes#718
Refs #723
Harden gitea_acquire_merger_pr_lease with required candidate_head, live-head
match, and fail-closed POST handling. Add capability-map aliases for reviewer
and merger lease acquire/adopt. Make unknown tasks return structured
unknown_task (no ValueError escape). Expand tests for roles, head scoping,
aliases, and unknown_task.
Closes#718. Addresses #723 resolver unknown_task and lease task mapping.
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.
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