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]>

task_capability_map.py

@@ -36,6 +36,12 @@ TASK_CAPABILITY_MAP: dict[str, dict[str, str]] = {
         "permission": "gitea.issue.comment",
         "role": "author",
     },
+    # #780: retire status:pr-open after a terminal PR transition. Same label
+    # authority as set_issue_labels — it is a strictly narrower operation.
+    "cleanup_terminal_pr_labels": {
+        "permission": "gitea.issue.comment",
+        "role": "author",
+    },
     "create_label": {
         "permission": "gitea.issue.comment",
         "role": "author",
@@ -507,6 +513,7 @@ ISSUE_MUTATION_TOOL_TASKS: dict[str, str] = {
     "gitea_create_issue_comment": "comment_issue",
     "gitea_mark_issue": "mark_issue",
     "gitea_set_issue_labels": "set_issue_labels",
+    "gitea_cleanup_terminal_pr_labels": "cleanup_terminal_pr_labels",
     "gitea_create_label": "create_label",
     "gitea_commit_files": "commit_files",
 }