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_tier3.py

@@ -235,3 +235,52 @@ class TestCli:
         err = capsys.readouterr().err
         assert rc == 10
         assert "[usage_error]" in err
+
+    @respx.mock
+    def test_cli_define_partial_response_degrades(
+        self,
+        capsys: pytest.CaptureFixture[str],
+        monkeypatch: pytest.MonkeyPatch,
+        _isolated_local_agents: "Path",
+    ) -> None:
+        """cli_define_partial [error]: a 201 missing `role` + a NULL system_prompt
+        degrades (role '?', description-safe) and exits 0 — never a KeyError/
+        AttributeError traceback (heid-bug-hunt open-world invariant)."""
+        from ratatoskr.local_agents import load_local_agents
+
+        monkeypatch.setenv("WORLDTREE_API_URL", "https://w.example")
+        monkeypatch.setenv("WORLDTREE_API_KEY", "k")
+        respx.post("https://w.example/agents/define").mock(
+            return_value=httpx.Response(201, json={
+                "agent_id": "ratatoskr:wizard", "agent_name": "wizard",
+                "system_prompt": None,  # present-but-null: .get(...,'') would NOT default
+            })
+        )
+        rc = main([
+            "define", "--name", "wizard", "--system-prompt", "x", "--role", "m",
+        ])
+        out = capsys.readouterr()
+        assert rc == 0
+        assert out.out.strip() == "defined ratatoskr:wizard (?)"
+        # Well-formed identity → still indexed (role degraded to '?').
+        entries = load_local_agents()
+        assert len(entries) == 1
+        assert entries[0].role == "?"
+
+    @respx.mock
+    def test_cli_define_no_agent_id_is_api_failure(
+        self, capsys: pytest.CaptureFixture[str], monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """cli_define_no_agent_id [error]: a 2xx with no usable agent_id → [api_failed]
+        + exit 20 (controlled), not an uncaught traceback."""
+        monkeypatch.setenv("WORLDTREE_API_URL", "https://w.example")
+        monkeypatch.setenv("WORLDTREE_API_KEY", "k")
+        respx.post("https://w.example/agents/define").mock(
+            return_value=httpx.Response(201, json={"role": "m"})  # no agent_id
+        )
+        rc = main([
+            "define", "--name", "wizard", "--system-prompt", "x", "--role", "m",
+        ])
+        err = capsys.readouterr().err
+        assert rc == 20
+        assert "[api_failed]" in err