feat(#19): ephemeral-template (Echo) session creation

create_session could only mint foundational sessions; an ephemeral template
(agent_id="echo") returned 422 ephemeral_requires_config because ratatoskr never
sent the required config block — Echo was uncreatable, surfacing as an opaque
session_api_failed at the CLI. Thread an opaque, role/model-agnostic config
passthrough through the create path so Echo sessions are creatable.

- sessions.py: create_session(config=...) verbatim passthrough (PRE-004 Mapping /
  PRE-005 config-xor-bifrost guards); SessionInfo gains kind + config, captured
  defensively (.get) on both create and list.
- cli.py: --system-prompt flag builds config={"system_prompt": ...} (validation:
  non-empty, requires --new+--agent, xor bifrost); _amain surfaces kind=; the
  --whoami renderer now reads allowed_roles/default_role (was reading the dead
  allowed_models/default_model) and tolerates a malformed capabilities shape.
- contract #2 amended (Amendment 2026-07-18); Heid-panel contract-reviewed +
  diff-scoped bug-hunted (one whoami null-join gap found + fixed).

Canonical grounding: config.role, never config.model (worldtree-dev althing
01KXT976NN91DRBZBPXNZ2BVZR; ADR-0012 role cutover). Verified end-to-end against
the live v0.16.2 target. TDD across create + CLI; full suite green (534).

Closes #19.

tests/test_cli.py

@@ -281,6 +281,45 @@ class TestParseArgs:
         )
         assert args.end_user_id == "from-flag"
 
+    def test_system_prompt_sets_field(self) -> None:
+        """system_prompt_sets_field [happy, #161]: --system-prompt → ParsedArgs.system_prompt."""
+        args = _parse_args(
+            ["--send", "hi", "--new", "--agent", "echo",
+             "--system-prompt", "You are X.", "--api-key", "k"]
+        )
+        assert args.system_prompt == "You are X."
+
+    def test_system_prompt_default_none(self) -> None:
+        """system_prompt_default_none [trace, #161]: omitted → None (foundational baseline)."""
+        args = _parse_args(["--send", "hi", "--new", "--agent", "mimir", "--api-key", "k"])
+        assert args.system_prompt is None
+
+    def test_system_prompt_empty_rejected(self) -> None:
+        """system_prompt_empty_rejected [adversarial, #161]: '' → UsageError."""
+        with pytest.raises(UsageError, match="--system-prompt"):
+            _parse_args(
+                ["--send", "hi", "--new", "--agent", "echo",
+                 "--system-prompt", "", "--api-key", "k"]
+            )
+
+    def test_system_prompt_requires_new(self) -> None:
+        """system_prompt_requires_new [adversarial, #161]: with --session → UsageError."""
+        with pytest.raises(UsageError, match="--system-prompt"):
+            _parse_args(
+                ["--send", "hi", "--session", "s-1",
+                 "--system-prompt", "You are X.", "--api-key", "k"]
+            )
+
+    def test_system_prompt_xor_bifrost(self) -> None:
+        """system_prompt_xor_bifrost [adversarial, #161]: config + bifrost → UsageError."""
+        with pytest.raises(UsageError, match="mutually exclusive"):
+            _parse_args(
+                ["--send", "hi", "--new", "--agent", "echo",
+                 "--system-prompt", "You are X.",
+                 "--bifrost-plane", "memory", "--bifrost-host", "h.example",
+                 "--api-key", "k"]
+            )
+
 
 SID = SseId(42, 5)
 
@@ -1225,6 +1264,45 @@ class TestAmain:
         assert "[done]" in err
         assert err.index(". create_session:") < err.index("[done]")
 
+    @respx.mock
+    async def test_ephemeral_create_sends_config(
+        self, capsys: pytest.CaptureFixture[str]
+    ) -> None:
+        """ephemeral_create_sends_config [#161]: --system-prompt → config sent; kind surfaced."""
+        import json as _json
+
+        sessions_route = respx.post("https://w.example/sessions").mock(
+            return_value=httpx.Response(
+                201,
+                json={
+                    **_CREATE_OK_RESP,
+                    "kind": "ephemeral",
+                    "config": {"system_prompt": "You are X.", "role": "echo"},
+                },
+            )
+        )
+        sse_body = _sse_chunk("42:1", {"type": "text", "content": "hi"}) + _sse_chunk(
+            "42:2", _DONE_BODY
+        )
+        respx.post("https://w.example/sessions/s-new/messages").mock(
+            return_value=_sse_resp(sse_body)
+        )
+        parsed = ParsedArgs(
+            send_content="hi",
+            session_id=None,
+            new=True,
+            agent_id="echo",
+            api_key="k",
+            server_url="https://w.example",
+            raw=False,
+            system_prompt="You are X.",
+        )
+        exit_code = await _amain(parsed)
+        assert exit_code == 0
+        body = _json.loads(sessions_route.calls[0].request.content)
+        assert body == {"agent_id": "echo", "config": {"system_prompt": "You are X."}}
+        assert "kind=ephemeral" in capsys.readouterr().err
+
     @respx.mock
     async def test_happy_existing_session(self, capsys: pytest.CaptureFixture[str]) -> None:
         """happy_existing_session: --session, no create POST; just SSE stream → exit 0."""
@@ -1601,8 +1679,10 @@ class TestWhoami:
                 json={
                     "ephemeral_templates": {
                         "echo": {
-                            "allowed_models": ["glm5-turbo"],
-                            "default_model": "glm5-turbo",
+                            # Canonical post-cutover shape (worldtree-dev althing
+                            # 2026-07-18; ADR-0012): roles, not models.
+                            "allowed_roles": ["echo"],
+                            "default_role": "echo",
                             "system_prompt_max_bytes": 32768,
                         }
                     }
@@ -1616,7 +1696,37 @@ class TestWhoami:
         assert "tier: user" in out
         assert "key_id: a1b2c3d4" in out
         assert "ephemeral_template echo" in out
-        assert "glm5-turbo" in out
+        assert "default=echo" in out
+        assert "roles=[echo]" in out
+
+    @respx.mock
+    def test_whoami_tolerates_malformed_capabilities(
+        self, capsys: pytest.CaptureFixture[str]
+    ) -> None:
+        """whoami null/malformed caps → no crash; renders defensively (bug-hunt Gróa#1/#2)."""
+        respx.get("https://w.example/me").mock(
+            return_value=httpx.Response(
+                200, json={"user_id": "u", "scopes": [], "tier": "user"}
+            )
+        )
+        # allowed_roles: null (explicit) would crash `", ".join(None)`; a non-mapping
+        # template value would crash `spec.get(...)`. Both must degrade, not abort.
+        respx.get("https://w.example/capabilities").mock(
+            return_value=httpx.Response(
+                200,
+                json={
+                    "ephemeral_templates": {
+                        "echo": {"allowed_roles": None, "system_prompt_max_bytes": 32768},
+                        "broken": None,
+                    }
+                },
+            )
+        )
+        rc = main(["--whoami", "--api-key", "k", "--server", "https://w.example"])
+        assert rc == 0
+        out = capsys.readouterr().out
+        assert "roles=[]" in out
+        assert "broken: (malformed)" in out
 
     @respx.mock
     def test_whoami_me_auth_failure_exits_20(self, capsys: pytest.CaptureFixture[str]) -> None: