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.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.21.12"
+version = "0.21.13"
 description = "Worldtree Conversation API debug console (web + headless CLI) — multi-pane observability"
 readme = "README.md"
 requires-python = ">=3.12"