feat(#20): agents/tier3 family onto the wt adapter + model→role fold (slice-4)

Cut ratatoskr's consumer agent-lifecycle routes over to worldtree-sdk
(issue #20 slice-4). Five routes now flow through `ratatoskr.wt` over the
SDK's `client.agents.*`, returning open-world dicts and mapping the SDK's
undiscriminated `ApiError` floor by route+(status,error_code) per INV-CUT-2:

- `list_agents`      → `agents.list`
- `get_persona_state`→ `agents.persona_state` (404 persona_not_configured /
                        404 agent_not_available / 403 auth_scope_denied)
- `define_agent`     → `agents.define` (429→Tier3QuotaExceeded(retry_after=0),
                        403→Tier3UserIdUnsupported, 422 layer_deferred→…)
- `patch_agent`      → `agents.patch`  (404→Tier3AgentNotFound, 422 field_not_mutable)
- `delete_agent`     → `agents.delete` (404→Tier3AgentNotFound; NOT hide-existence)

Rewired call-sites: the `python -m ratatoskr.tier3` CLI (define/patch/delete)
and the web `_agents_endpoint` / `_persona_state_endpoint`, both catching the
SDK's `ConnectFailed` transport-failure normalization. Deleted the hand-rolled
paths: `sessions.list_agents` / `get_persona_state` / `AgentInfo`, and
`tier3.define/patch/delete_agent` / `Tier3AgentInfo` / parse+extract helpers.

model→role fold (scope B): the define/patch response echoes `role` (spec 1.2 /
b128), read off the open-world dict; `LocalAgentEntry.model`→`.role`,
local-index schema v1→2 (old index discarded, no-backwards-compat).

The Tier-3 caller-semantic exceptions move to `sessions.py`: running the CLI
as `__main__` while `wt` imports `ratatoskr.tier3` bound two copies of each
exception class, so a raised `Tier3AgentNotFound` escaped the CLI's `except`
as an uncaught traceback. Homing them in `sessions` (never `__main__`) makes
the class identity single. The live smoke — not the unit tests, which call
`main()` in-process — caught this.

Error-map rows + slice-4 notes added to the cutover contract; coverage-map
re-anchored. LIVE-SMOKE on personal :8081 (b128): define(thoughtful-character)
→ patch → list(6 agents) → persona_state(→PersonaNotConfigured mapped) →
delete → index empty; non-existent-id patch via `-m` → [agent_not_found]
exit 20. Suite 465 green.

tests/test_local_agents.py

@@ -33,14 +33,14 @@ def local_path(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
 def _entry(
     agent_id: str = "ratatoskr:wizard",
     agent_name: str = "wizard",
-    model: str = "qwen3.6-35-a3b",
+    role: str = "qwen3.6-35-a3b",
     description: str = "(tier 3) test agent",
     defined_at: str = "2026-05-25T00:00:00+00:00",
 ) -> LocalAgentEntry:
     return LocalAgentEntry(
         agent_id=agent_id,
         agent_name=agent_name,
-        model=model,
+        role=role,
         description=description,
         defined_at=defined_at,
     )
@@ -91,13 +91,13 @@ class TestLoadEmpty:
         local_path.write_text(
             json.dumps(
                 {
-                    "version": 1,
+                    "version": 2,
                     "agents": [
                         {"agent_id": "incomplete"},  # missing required fields
                         {
                             "agent_id": "ratatoskr:good",
                             "agent_name": "good",
-                            "model": "m",
+                            "role": "m",
                             "description": "d",
                             "defined_at": "t",
                         },
@@ -124,11 +124,11 @@ class TestAdd:
         assert ids == {"ratatoskr:a", "ratatoskr:b"}
 
     def test_add_replaces_same_id(self, local_path: Path) -> None:
-        add_local_agent(_entry(model="old-model"))
-        add_local_agent(_entry(model="new-model"))
+        add_local_agent(_entry(role="old-role"))
+        add_local_agent(_entry(role="new-role"))
         entries = load_local_agents()
         assert len(entries) == 1
-        assert entries[0].model == "new-model"
+        assert entries[0].role == "new-role"
 
     def test_creates_parent_dirs(
         self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
@@ -141,11 +141,11 @@ class TestAdd:
 
 class TestUpdate:
     def test_update_changes_existing(self, local_path: Path) -> None:
-        add_local_agent(_entry(model="v1"))
-        update_local_agent(_entry(model="v2"))
+        add_local_agent(_entry(role="v1"))
+        update_local_agent(_entry(role="v2"))
         entries = load_local_agents()
         assert len(entries) == 1
-        assert entries[0].model == "v2"
+        assert entries[0].role == "v2"
 
 
 class TestRemove: