Add Stop-B authority: deterministic validator overrides LLM stop

Phase 4 of the two-stop work. The harness already pre-empts the LLM with
deterministic stops and guided probes, but an LLM-originated should_stop could
still end the loop while the validator saw remaining opportunity.

Add harness._stop_authority, exposed as context["stop_authority"], whose
`authorized` mirrors the deterministic harness stop decision and whose
`opportunity_remains` flags an open topology frontier or a high-value planned
candidate. In study tune, an LLM-originated should_stop is now honored only when
the validator authorizes it; an unauthorized stop is vetoed (bounded budget) so
the loop cannot converge prematurely on the agent's say-so. File- and
harness-originated stops are unaffected, and the stop reason chain is recorded.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/aituner/cli.py

@@ -226,6 +226,8 @@ def cmd_study_tune(args: argparse.Namespace) -> int:
     if proposal_files and max_trials > len(proposal_files):
         max_trials = len(proposal_files)
     executed: list[dict[str, object]] = []
+    stop_vetoes = 0
+    max_llm_stop_vetoes = 1
     for idx in range(max_trials):
         state = store.load_state(study.study_id)
         if state.tuning_stop_reason:
@@ -334,7 +336,34 @@ def cmd_study_tune(args: argparse.Namespace) -> int:
         proposal = parse_proposal_text(proposal_text, study)
         store.write_proposal(study.study_id, proposal_name, proposal)
         if proposal.should_stop:
-            if proposal_name.startswith("harness-stop-"):
+            is_harness_stop = proposal_name.startswith("harness-stop-")
+            is_llm_stop = not is_harness_stop and proposal_source is None
+            stop_authority = (
+                harness_context.get("stop_authority")
+                if isinstance(harness_context, dict)
+                else None
+            )
+            authorized = stop_authority is None or bool(stop_authority.get("authorized"))
+            # Stop-B authority: the deterministic validator overrides an
+            # LLM-originated stop. Veto an unauthorized stop (bounded) so the
+            # loop does not converge prematurely on the agent's say-so alone.
+            if is_llm_stop and not authorized and stop_vetoes < max_llm_stop_vetoes:
+                stop_vetoes += 1
+                executed.append(
+                    {
+                        "trial_id": None,
+                        "proposal_name": proposal_name,
+                        "proposal_source": "llm",
+                        "stop_vetoed": True,
+                        "reason": "validator_did_not_authorize_stop",
+                        "validator_reason": (
+                            stop_authority.get("reason") if stop_authority else None
+                        ),
+                        "diagnosis": proposal.diagnosis,
+                    }
+                )
+                continue
+            if is_harness_stop:
                 proposal_source_label = "harness"
             else:
                 proposal_source_label = str(proposal_source) if proposal_source else "llm"
@@ -344,6 +373,11 @@ def cmd_study_tune(args: argparse.Namespace) -> int:
                     "proposal_name": proposal_name,
                     "proposal_source": proposal_source_label,
                     "stopped": True,
+                    "stop_authorized_by": (
+                        "validator"
+                        if (is_harness_stop or authorized)
+                        else "llm_after_veto_budget"
+                    ),
                     "diagnosis": proposal.diagnosis,
                     "state_best_trial_id": state.best_trial_id,
                     "state_best_request_rate": state.best_request_rate,