feat(sessions,cli,tui): issue #8 — startup agent picker (v0.3.0)

Adds GET /agents fetch + ListView picker for bare `--new` (TUI mode
without --agent). Three in-place amendments:

- ratatoskr.sessions: new `list_agents()` + `AgentInfo` frozen
  dataclass with omit-when-null/empty defaults mirroring SessionInfo's
  INV-001/INV-002 origin-conditional pattern. Non-200 responses raise
  the existing SessionApiFailed (no new exception).
- ratatoskr.cli: `_parse_args` softens `--agent` from absolute to
  mode-conditional — required for `--send --new`, optional for bare
  `--new`, forbidden with `--session` (unchanged INV-004).
- ratatoskr.tui: new `AgentPickerApp(App[str | None])` — separate
  Textual App (not Screen-within-RatatoskrApp) so list_agents errors
  land on real stderr before any alt-screen opens (preserves issue
  #6's INV-001). `_resolve_then_run` gains a pre-create branch:
  fetch agents → empty list → exit 13; non-200 → exit 20; network
  error → exit 21; picker dismissed → exit 0; otherwise thread chosen
  agent_id into create_session.

Contract: docs/contracts/issues/8.contract.md (drift-check clean).

Tests: +18 (227 total, was 209). Live smoke against personal Worldtree
(:8081) returned 12 agents; programmatic picker drive auto-picked lofn
and created a real session with `end_user_id="ratatoskr-tui"`.

tests/test_sessions.py

@@ -5,11 +5,13 @@ import pytest
 import respx
 
 from ratatoskr.sessions import (
+    AgentInfo,
     AgentNotFound,
     InvalidCursor,
     SessionApiFailed,
     SessionPage,
     create_session,
+    list_agents,
     list_sessions,
 )
 
@@ -412,3 +414,145 @@ class TestListSessions:
             with pytest.raises(AssertionError):
                 await list_sessions(client, cursor="")
         assert route.call_count == 0
+
+
+# ---- Issue #8: list_agents + AgentInfo --------------------------------------
+
+
+class TestListAgents:
+    @respx.mock
+    async def test_happy_full_shape(self) -> None:
+        """happy_full_shape [happy,tracer]: spec full-shape mimir example → all fields."""
+        respx.get("https://w.example/agents").mock(
+            return_value=httpx.Response(
+                200,
+                json=[
+                    {
+                        "agent_id": "mimir",
+                        "name": "Mimir",
+                        "description": "Keeper of the Well of Knowledge.",
+                        "version": "0.2.0",
+                        "capabilities": ["knowledge_base", "semantic_search"],
+                        "supported_models": ["default", "heavy"],
+                        "persona_traits": {
+                            "ocean": {
+                                "openness": 0.7,
+                                "conscientiousness": 0.9,
+                                "extraversion": 0.1,
+                                "agreeableness": 0.5,
+                                "neuroticism": 0.3,
+                            },
+                            "vibe": "contemplative",
+                        },
+                        "ui_hints": {"icon": "well", "color_hint": "#5b8aa3"},
+                    }
+                ],
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            agents = await list_agents(client)
+        assert len(agents) == 1
+        a = agents[0]
+        assert isinstance(a, AgentInfo)
+        assert a.agent_id == "mimir"
+        assert a.name == "Mimir"
+        assert a.description == "Keeper of the Well of Knowledge."
+        assert a.version == "0.2.0"
+        assert a.capabilities == ["knowledge_base", "semantic_search"]
+        assert a.supported_models == ["default", "heavy"]
+        assert a.persona_traits["vibe"] == "contemplative"
+        assert a.ui_hints["icon"] == "well"
+
+    @respx.mock
+    async def test_happy_minimum_shape(self) -> None:
+        """happy_minimum_shape: required-only agent → optional fields default."""
+        respx.get("https://w.example/agents").mock(
+            return_value=httpx.Response(
+                200,
+                json=[
+                    {
+                        "agent_id": "minimal",
+                        "name": "Minimal Agent",
+                        "description": "Just a sketch.",
+                    }
+                ],
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            agents = await list_agents(client)
+        a = agents[0]
+        assert a.agent_id == "minimal"
+        assert a.version is None
+        assert a.capabilities == []
+        assert a.supported_models == []
+        assert a.persona_traits == {}
+        assert a.ui_hints == {}
+
+    @respx.mock
+    async def test_happy_multi_agent(self) -> None:
+        """happy_multi_agent: 3 agents preserve order."""
+        respx.get("https://w.example/agents").mock(
+            return_value=httpx.Response(
+                200,
+                json=[
+                    {"agent_id": "a", "name": "A", "description": "x"},
+                    {"agent_id": "b", "name": "B", "description": "y"},
+                    {"agent_id": "c", "name": "C", "description": "z"},
+                ],
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            agents = await list_agents(client)
+        assert [a.agent_id for a in agents] == ["a", "b", "c"]
+
+    @respx.mock
+    async def test_happy_empty(self) -> None:
+        """happy_empty: 200 with [] returns empty list (no error)."""
+        respx.get("https://w.example/agents").mock(
+            return_value=httpx.Response(200, json=[])
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            agents = await list_agents(client)
+        assert agents == []
+
+    @respx.mock
+    async def test_omit_capabilities_empty_list(self) -> None:
+        """omit_capabilities_empty: explicit [] from server still defaults to []."""
+        respx.get("https://w.example/agents").mock(
+            return_value=httpx.Response(
+                200,
+                json=[
+                    {
+                        "agent_id": "a",
+                        "name": "A",
+                        "description": "x",
+                        "capabilities": [],
+                    }
+                ],
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            agents = await list_agents(client)
+        assert agents[0].capabilities == []
+
+    @respx.mock
+    async def test_500_raises_session_api_failed(self) -> None:
+        """500 → SessionApiFailed with status=500."""
+        respx.get("https://w.example/agents").mock(
+            return_value=httpx.Response(500, content=b"oops")
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SessionApiFailed) as excinfo:
+                await list_agents(client)
+        assert excinfo.value.status == 500
+
+    @respx.mock
+    async def test_401_raises_session_api_failed(self) -> None:
+        """401 → SessionApiFailed with status=401."""
+        respx.get("https://w.example/agents").mock(
+            return_value=httpx.Response(401, content=b'{"error":"unauthorized"}')
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SessionApiFailed) as excinfo:
+                await list_agents(client)
+        assert excinfo.value.status == 401