fix(#20): heid-bug-hunt fixups — open-world presenter degrade-not-crash (slice-4)

Panel (Gróa+Hulda+Regin, 5/5/5, no false positives) confirmed two 3/3 crash
sites where open-world dict reads violate the declared "degrade, never crash the
presenter" invariant — the wt adapter tests + the live smoke used full server
dicts, so partial/drifted wire responses were never exercised:

- FIX (tier3.py _run_define/_run_patch): the CLI hard-indexed the open-world
  define/patch dicts (`info["agent_id"]` / `["role"]` / `["agent_name"]`), so a
  partial 2xx → KeyError escaping main()'s exit matrix as a raw traceback (exit 1);
  and `make_description(info.get("system_prompt", ""))` fed None to .splitlines()
  on a present-but-null field → AttributeError. Now reads via `_str_field` (absent/
  null/non-str → default), degrades role to '?', indexes only a well-formed identity,
  and maps a no-usable-agent_id 2xx to [api_failed] exit 20 (controlled, not a crash).
- FIX (web/server.py _agents_endpoint): the upstream dedup hard-indexed each item
  (`{a["agent_id"] for a in upstream}` + `_as_dict`), so a malformed item (`[{}]`,
  `["str"]`, `{"name":…}`, non-str agent_id) or a non-list envelope → 500 before the
  local fallback merged. Now filters to well-formed mappings first; a non-list
  upstream degrades to the local-only list.
- FIX (wt.py _error_field_from_body): type-check the parsed `field` is a str (the
  exception surface is `field: str | None`, the CLI prints it) — restores the retired
  hand-rolled `_extract_error_field` isinstance guard.

Held (triaged, no change): the 429→Tier3QuotaExceeded / bare-404→Tier3AgentNotFound
maps are ungated-by-error_code BY CONTRACT DESIGN (§ Error map route+status rows; the
SDK's ApiError floor drops Retry-After, so retry_after=0 is canonical) — the arms
flagged them spec-free; Heid's source-check confirmed intended. Dual-keying define's
429 for full row consistency is an available tightening (contract amendment), surfaced
not applied. The persona-endpoint SessionApiFailed gap the arms also caught was
already closed in the prior code-review fixup (aed9429).

Suite 475 green (+5).

tests/test_wt.py

@@ -705,6 +705,16 @@ class TestDefineAgent:
             await define_agent(_wta(fake), agent_name="wizard", system_prompt="x", role="m")
         assert ei.value.field == "valence"
 
+    async def test_layer_deferred_non_string_field_is_none(self) -> None:
+        # A non-string `field` value collapses to None — the exception surface is
+        # `field: str | None` and the CLI prints it (heid-bug-hunt hardening).
+        fake = _FakeAgents(error=ApiError(
+            "layer_deferred", "no", status=422, body='{"detail": {"field": {"x": 1}}}',
+        ))
+        with pytest.raises(Tier3LayerDeferred) as ei:
+            await define_agent(_wta(fake), agent_name="wizard", system_prompt="x", role="m")
+        assert ei.value.field is None
+
     async def test_403_wrong_code_maps_to_default(self) -> None:
         # Dual-key negative: a 403 whose code is NOT tier3_user_id_unsupported →
         # generic default, not a spurious Tier3UserIdUnsupported (INV-CUT-2).