feat(local_agents): tier-3 index + picker merge (v0.8.0)

Worldtree's GET /agents doesn't return consumer-defined (tier-3)
agents — the public list excludes them by design. Confirmed live in
v0.7.0's smoke. Without server-side knowledge, ratatoskr's picker
couldn't show tier-3 agents the operator had defined; the workflow
was "remember the agent_id, pass --agent ratatoskr:<name>
explicitly." Friction grows with every tier-3 agent.

## Fix: client-side index, merged at picker time

New module `ratatoskr.local_agents` maintains a JSON-backed index at
$XDG_CONFIG_HOME/ratatoskr/local_agents.json (override via
$RATATOSKR_LOCAL_AGENTS). `tier3` CLI define / patch / delete update
the index as side-effects. `tui._resolve_then_run` loads the index
after `list_agents(client)` and appends entries not already in the
remote list (dedup by agent_id; remote wins on conflict).

Library-level `tier3.define_agent` / `patch_agent` / `delete_agent`
stay pure — local persistence lives in the CLI layer (`_run_define`
etc.), not in the library functions. Tests of the library don't
touch the filesystem.

## Public surface

  ratatoskr.local_agents:
    LocalAgentEntry (frozen dataclass)
    load_local_agents() -> list[LocalAgentEntry]
    add_local_agent(entry)
    update_local_agent(entry)  # same semantics as add (agent_id key)
    remove_local_agent(agent_id)
    make_description(system_prompt) -> str  # synthetic picker label

Failure modes are lenient: missing file → empty index; corrupt JSON
or schema mismatch → empty index (no crash). The picker continues
to show foundational agents either way; tier-3 surface degrades to
the pre-v0.8.0 workflow.

## Picker integration

Local entries convert to ratatoskr.sessions.AgentInfo with synthetic
fields:
  name        = agent_name (from LocalAgentEntry)
  description = "(tier 3) <first non-empty line of system prompt>"
  version, capabilities, supported_models, persona_traits, ui_hints
    = None / [] / [] / {} / {}

If Worldtree later starts returning tier-3 in GET /agents, this
module's role narrows to redundant local cache; can be removed
cleanly since the dedup-by-agent-id keeps remote-wins behavior.

## Tests

286/286 GREEN (was 265, +21: 20 local_agents + 1 picker-merge
integration). Ruff clean. Tests isolate the index via
$RATATOSKR_LOCAL_AGENTS pointed at pytest's tmp_path — no pollution
of operator's real ~/.config/ratatoskr/.

## Manual smoke

Sindra-like define against personal Worldtree:
  python -m ratatoskr.tier3 define --name foo --system-prompt "..." --model X
  cat ~/.config/ratatoskr/local_agents.json
  # ratatoskr --new picker now shows ratatoskr:foo alongside mimir et al.

Cross-machine: the file is per-host. Operator can sync via dotfiles
if needed; out of scope for this commit.

Minor bump (v0.7.1 → v0.8.0) — new public module + new picker
behavior (more agents shown). No caller-side breaking changes.

tests/test_tier3.py

@@ -1,5 +1,7 @@
 """Tests for ratatoskr.tier3 per docs/contracts/issues/15.contract.md."""
 
+from pathlib import Path
+
 import httpx
 import pytest
 import respx
@@ -315,12 +317,29 @@ class TestDeleteAgent:
         assert exc.value.status == 500
 
 
+@pytest.fixture
+def _isolated_local_agents(
+    tmp_path: "Path", monkeypatch: pytest.MonkeyPatch
+) -> "Path":
+    """Isolate the v0.8.0 local-tier-3 index from the operator's real file."""
+    path = tmp_path / "local_agents.json"
+    monkeypatch.setenv("RATATOSKR_LOCAL_AGENTS", str(path))
+    return path
+
+
 class TestCli:
     @respx.mock
     def test_cli_define_happy(
-        self, capsys: pytest.CaptureFixture[str], monkeypatch: pytest.MonkeyPatch
+        self,
+        capsys: pytest.CaptureFixture[str],
+        monkeypatch: pytest.MonkeyPatch,
+        _isolated_local_agents: "Path",
     ) -> None:
-        """cli_define_happy [happy]: argv → 201 mock → stdout confirmation."""
+        """cli_define_happy [happy]: argv → 201 mock → stdout confirmation;
+        local index updated with the new entry (v0.8.0 hook).
+        """
+        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(
@@ -335,12 +354,24 @@ class TestCli:
         out = capsys.readouterr()
         assert rc == 0
         assert out.out.strip() == "defined ratatoskr:wizard (qwen3.6-35-a3b)"
+        # v0.8.0: local index now has the new entry.
+        entries = load_local_agents()
+        assert len(entries) == 1
+        assert entries[0].agent_id == "ratatoskr:wizard"
+        assert entries[0].model == "qwen3.6-35-a3b"
 
     @respx.mock
     def test_cli_patch_happy(
-        self, capsys: pytest.CaptureFixture[str], monkeypatch: pytest.MonkeyPatch
+        self,
+        capsys: pytest.CaptureFixture[str],
+        monkeypatch: pytest.MonkeyPatch,
+        _isolated_local_agents: "Path",
     ) -> None:
-        """cli_patch_happy [happy]: argv → 200 mock → stdout confirmation."""
+        """cli_patch_happy [happy]: argv → 200 mock → stdout confirmation;
+        local index refreshed with the post-patch state.
+        """
+        from ratatoskr.local_agents import load_local_agents
+
         monkeypatch.setenv("WORLDTREE_API_URL", "https://w.example")
         monkeypatch.setenv("WORLDTREE_API_KEY", "k")
         respx.patch("https://w.example/agents/ratatoskr:wizard").mock(
@@ -350,12 +381,34 @@ class TestCli:
         out = capsys.readouterr()
         assert rc == 0
         assert out.out.strip() == "patched ratatoskr:wizard"
+        entries = load_local_agents()
+        assert len(entries) == 1
+        assert entries[0].agent_id == "ratatoskr:wizard"
 
     @respx.mock
     def test_cli_delete_happy(
-        self, capsys: pytest.CaptureFixture[str], monkeypatch: pytest.MonkeyPatch
+        self,
+        capsys: pytest.CaptureFixture[str],
+        monkeypatch: pytest.MonkeyPatch,
+        _isolated_local_agents: "Path",
     ) -> None:
-        """cli_delete_happy [happy]: argv → 204 mock → stdout confirmation."""
+        """cli_delete_happy [happy]: argv → 204 mock → stdout confirmation;
+        local index entry removed (v0.8.0 hook).
+        """
+        from ratatoskr.local_agents import (
+            LocalAgentEntry,
+            add_local_agent,
+            load_local_agents,
+        )
+
+        # Pre-populate so we can verify removal.
+        add_local_agent(LocalAgentEntry(
+            agent_id="ratatoskr:wizard",
+            agent_name="wizard",
+            model="m",
+            description="d",
+            defined_at="t",
+        ))
         monkeypatch.setenv("WORLDTREE_API_URL", "https://w.example")
         monkeypatch.setenv("WORLDTREE_API_KEY", "k")
         respx.delete("https://w.example/agents/ratatoskr:wizard").mock(
@@ -365,6 +418,7 @@ class TestCli:
         out = capsys.readouterr()
         assert rc == 0
         assert out.out.strip() == "deleted ratatoskr:wizard"
+        assert load_local_agents() == []
 
     def test_cli_missing_auth(
         self, capsys: pytest.CaptureFixture[str], monkeypatch: pytest.MonkeyPatch