pr-merged

The closing phase of one sub-task: once its pull request has merged, record the completion and work out which pending sub-tasks that releases. Verifies the merge by asking GitHub rather than trusting the claim, writes a completion record stamped verified: github, recomputes the released set, and hands that set back to its caller. Iteration belongs to /advance, not to this phase. Use when: the loop closes a sub-task, or a person says /pr-merged, 'PR 40 is merged', 'these PRs merged, continue', or returns after merging work outside a running loop. Reads a contract handoff plan and writes the result and state stores.