feat(#2): Tier-2 — transient characters + persona-state write; audit converges

v1 coverage-audit: the last in-scope client I/O points. The audit now
CONVERGES — REST 17/40 covered with zero in-scope gaps (23 excluded-by-
design), SSE 11/11, Bifrost planes 8/8.

- sessions.py: list_character_models / create_character / get_character_state
  / delete_character (#161, character.read/write) + set_persona_state
  (POST /sessions/{id}/persona_state — freeform body, unpinned in the
  frozen surface). 200/201 -> dict (or None on 204), off-status ->
  SessionApiFailed.
- cli.py: two one-shot probes (mirror --whoami): --characters (CRUD
  lifecycle report) + --set-persona-pad "p,a,d" (requires --session).
  New ParsedArgs.characters/set_persona_pad + probe mutual-exclusion.
- Contract #2 amended (5 FNs) + validated. TDD: 7 wrapper + 5 cli tests.
  Suite 573 green; touched code ruff-clean.
- Char read side live-proven (GET /models/available-for-characters -> 200).

Coverage-map: convergence frontier CLOSED — scope-A "done" (every frozen
I/O point classified) is met; ratatoskr cuts v1 when Worldtree tags 1.0.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.19.0"
+version = "0.19.1"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"