fix(workflow): retire status:pr-open on every terminal PR transition (Closes #780)

status:pr-open was applied by gitea_create_pr and never removed again. Every
terminal path finished without touching it, so a repository audit found 40
closed issues still advertising an open PR.

Add terminal_pr_label_cleanup.py as the single authoritative rule and route
every sanctioned terminal path through it, so the paths cannot drift:

- merge (gitea_merge_pr)
- close without merge (gitea_edit_pr)
- supersession/abandonment (gitea_reconcile_superseded_by_merged_pr)
- already-landed reconciliation (gitea_reconcile_already_landed_pr)
- controller closure (gitea_close_issue)
- retry/recovery (new gitea_cleanup_terminal_pr_labels)

The rule removes only status:pr-open, preserves every other label, allows an
empty resulting set, is a no-op when the label is absent (so retries are
safe), and confirms the outcome by read-after-write rather than assumption.

Controller closure runs the cleanup before the state change and fails closed
if it cannot be completed and verified; closing first would bake in the stale
label with no later step to catch it. Post-merge cleanup never blocks the
merge, which already happened, and reports failures with a safe next action.

Also:
- gitea_assess_terminal_label_hygiene: read-only terminal validation that
  reports residual status:pr-open, exempting issues with a genuinely open PR.
- _put_issue_label_names now accepts Gitea's empty response body when the
  requested set is empty, so clearing the last label works.
- test_audit's close_issue fixture keys on the request instead of call order,
  since closing now also reads labels for the cleanup and its read-back.

Docs: label-taxonomy terminal-transition section, runbook pointer, and the
review-merge / reconcile-landed final-report terminal-label requirements.

Suite: 4045 passed, 11 failed, 6 skipped. The same 11 failures reproduce on
clean master df31674 (4010 passed, 11 failed) and are pre-existing.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

docs/llm-workflow-runbooks.md

@@ -706,7 +706,9 @@ do **not** improvise shell wrappers or fall back to direct API / temp scripts.
   `fix/...` / `docs/...`); `cd` into that worktree; implement narrowly; add or
   update tests if behavior changes; run the full suite; commit with an
   issue-linked message; open a PR to `master`; move the issue to
-  `status:pr-open`. **Do not** review or merge your own PR. Include an
+  `status:pr-open` (every terminal transition later retires that label
+  automatically — see [`label-taxonomy.md`](label-taxonomy.md)). **Do not**
+  review or merge your own PR. Include an
   `LLM Handoff Metadata` block (with `LLM-Agent-SHA`) in the PR body — see
   [`llm-agent-sha.md`](llm-agent-sha.md).
 - **Prompt:** `Use an author profile to implement issue #N and open a PR to