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"`.

tests/test_cli.py

@@ -176,11 +176,37 @@ class TestParseArgs:
         with pytest.raises(UsageError, match="pass exactly one"):
             _parse_args(["--send", "hi", "--api-key", "k"])
 
-    def test_usage_new_without_agent(self) -> None:
-        """usage_new_without_agent: --new without --agent → UsageError."""
+    def test_usage_send_new_without_agent(self) -> None:
+        """send_new_without_agent (issue #8): --send --new without --agent → UsageError.
+
+        --send mode is non-interactive — cannot prompt; --agent stays required.
+        """
         with pytest.raises(UsageError, match="--agent is required when --new"):
             _parse_args(["--send", "hi", "--new", "--api-key", "k"])
 
+    def test_parse_bare_new_without_agent_accepted(self) -> None:
+        """bare_new_without_agent (issue #8): --new without --send or --agent → agent_id=None.
+
+        TUI mode CAN prompt; startup picker handles the choice. _parse_args
+        accepts None here and the TUI's _resolve_then_run drives the picker.
+        """
+        args = _parse_args(["--new", "--api-key", "k"])
+        assert args.new is True
+        assert args.agent_id is None
+        assert args.send_content is None
+
+    def test_parse_bare_new_with_agent_accepted(self) -> None:
+        """bare_new_with_agent (issue #8): --new --agent mimir (no --send) → picker skipped.
+
+        Existing TUI launch path with an explicit agent_id continues to work
+        — _resolve_then_run sees `args.agent_id is not None` and skips the
+        picker entirely.
+        """
+        args = _parse_args(["--new", "--agent", "mimir", "--api-key", "k"])
+        assert args.new is True
+        assert args.agent_id == "mimir"
+        assert args.send_content is None
+
     def test_usage_session_with_agent(self) -> None:
         """usage_session_with_agent: --session AND --agent → UsageError."""
         with pytest.raises(UsageError, match="forbidden with --session"):