fix(tests): hermetic ORCH-41 model/effort tests vs host env (unblock merge-gate)

Merge-gate re-test runs under the orchestrator's prod env, where the
operator legitimately set ORCH_AGENT_FALLBACK_MODEL and changed
ORCH_AGENT_MODEL_DEFAULT / ORCH_AGENT_EFFORT_*. Two ORCH-41-era tests
asserted SHIPPED defaults through the env-backed settings singleton and
failed 3/3 there, while Gitea CI (clean env) stayed green. Branch
ORCH-009 touches neither src/ nor these tests - latent non-hermetic
landmine on main, detonated by the prod env change.

- test_resolve_agent_effort.py: autouse fixture now mirrors the sibling
  model-file baseline (pins shipped model/fallback fields) so the
  flag-assembly tests are env-independent.
- test_resolve_agent_model.py: fixture also resets agent_fallback_model;
  test_fallback_model_disabled_by_default now asserts the CLASS field
  default (the actual ORCH-074 ADR-001 G4 invariant: shipped default
  is ""), never-break is_valid_model asserts unchanged byte-for-byte.

Clean-env behaviour is byte-equivalent (fixtures pin exactly what an
empty env yields). Full suite: 1713 passed (was 2 failed / 1711).

Refs: ORCH-009

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

tests/test_resolve_agent_effort.py

@@ -42,6 +42,15 @@ def _clean_settings(monkeypatch):
     monkeypatch.setattr(settings, "agent_effort_default", "high")
     for a, e in CANON_EFFORT.items():
         monkeypatch.setattr(settings, f"agent_effort_{a}", e)
+    # Hermeticity (mirrors test_resolve_agent_model's baseline): the flag-assembly
+    # tests below also read the MODEL/fallback fields, and the host env (prod .env;
+    # the merge-gate re-test runs under it) may legitimately set
+    # ORCH_AGENT_MODEL_* / ORCH_AGENT_FALLBACK_MODEL. These tests assert
+    # shipped-default behaviour, not the host config -> pin the shipped defaults.
+    monkeypatch.setattr(settings, "agent_model_default", "claude-opus-4-8")
+    for a in CANON_EFFORT:
+        monkeypatch.setattr(settings, f"agent_model_{a}", "")
+    monkeypatch.setattr(settings, "agent_fallback_model", "")
     monkeypatch.setattr(P.settings, "projects_json", "")
     reload_projects()
     yield