[bug] pr_review_claim: set active_claimant_id so the reviewer can post the review

pr_review_claim set assigned_to + claimed_by + heartbeat but never
active_claimant_id (the single-claimant invariant _qa_or_doc_claim /
_finalize_claim set). So after claim_pr_review the reviewer could read
the PR but every note()/evidence() hit _active_claim_violation ->
_not_active_claimant (not_authorized): the journal:learning entry
post_pr_review's tracing gate requires could never be recorded, the
reviewer deadlocked at post_pr_review (tracing_gap), burned tokens into
the do-server breaker, idled -> task stuck paused (ccfb1bd7).

Set active_claimant_id + claimed_at at claim (mirrors QA/doc); cleared by
complete_review. TDD: extended test_pr_review_claim_and_complete to assert
the invariant (set at claim, cleared at complete); 35 pr_review tests +
ruff/mypy green.

roboco/services/task.py

@@ -1386,8 +1386,18 @@ class TaskService(BaseService):
         task = await self.get(task_id)
         if task is None or task.status != TaskStatus.PENDING:
             return None
+        now = datetime.now(UTC)
         task.assigned_to = cast("Any", reviewer_agent_id)
         task.claimed_by = cast("Any", reviewer_agent_id)
+        task.claimed_at = now
+        # Single-claimant invariant (mirrors _qa_or_doc_claim / _finalize_claim):
+        # active_claimant_id is what _active_claim_violation checks for content
+        # writes (note/evidence). Without it the reviewer can claim + read the PR
+        # but every note() returns _not_active_claimant, so the journal:learning
+        # entry post_pr_review's tracing gate requires can never be recorded ->
+        # the reviewer deadlocks at post_pr_review (tracing_gap) and burns tokens
+        # into the do-server breaker. Cleared by complete_review.
+        task.active_claimant_id = cast("Any", reviewer_agent_id)
         self._validate_and_set_status(
             task, TaskStatus.CLAIMED, "pr_reviewer", audit_agent_id=reviewer_agent_id
         )

tests/unit/services/test_pr_review_db.py

@@ -243,6 +243,16 @@ async def test_pr_review_claim_and_complete(db_session: AsyncSession) -> None:
     # claim to the reaper and, for a GROK reviewer, trips the idle-kill
     # watchdog before the review is posted (the wedge/respawn loop).
     assert claimed.last_heartbeat_at is not None
+    # Single-claimant invariant (mirrors _qa_or_doc_claim / _finalize_claim):
+    # active_claimant_id is what _active_claim_violation checks for content
+    # writes (note/evidence). Without it the reviewer can claim + read the PR
+    # but every note() returns _not_active_claimant -> the journal:learning
+    # entry post_pr_review's tracing gate requires can never be recorded ->
+    # the reviewer deadlocks at post_pr_review (tracing_gap) and burns tokens
+    # into the do-server breaker. claimed_at is set for parity with QA/doc.
+    assert claimed.active_claimant_id is not None
+    assert UUID(str(claimed.active_claimant_id)) == reviewer_id
+    assert claimed.claimed_at is not None
 
     # Re-claiming a non-pending task is a no-op.
     assert await svc.pr_review_claim(reviewer_id, task_id) is None
@@ -257,6 +267,8 @@ async def test_pr_review_claim_and_complete(db_session: AsyncSession) -> None:
     assert done.qa_notes is None
     assert (done.notes_structured or {}).get("pr_review", {}).get("verdict")
     assert done.claimed_by is None
+    # Single-claimant lock cleared on completion (the review hand-off is done).
+    assert done.active_claimant_id is None
 
     # Re-completing a completed task is a no-op.
     assert await svc.complete_review(reviewer_id, task_id) is None