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"`.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.2.1"
+version = "0.3.0"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"