feat(#6): startup session picker for bare TUI mode

v1 coverage-audit slice b2. The audit found list_sessions had no
caller — the startup session picker (design-brief §4) was never built;
bare TUI mode was a hard usage error. Add SessionPickerApp (mirrors
AgentPickerApp) and resolve bare mode in _resolve_then_run.

- Bare TUI mode (no --session/--new) now valid → session picker.
  Resolution: 0 sessions -> [no_sessions] exit 14 (resume-only per
  §4 "no in-app creation, --new only"); exactly 1 -> auto-resume
  (§4 "picker only when >1"); >=2 -> SessionPickerApp -> resume pick
  (Esc/Ctrl-D -> exit 0).
- cli._parse: bare TUI valid; --send still requires one flag; --agent
  forbidden in bare mode. run_tui PRE-002 xor -> mutually-exclusive.
- Contract #6 amended (SessionPickerApp + bare-mode resolution) +
  validated. TDD: 3 picker pilot tests + 5 resolution tests + 3 cli
  validation tests. Suite 528 green; touched code ruff-clean.

Design note: bare + 0 sessions errors (honors §4's no-in-app-creation
clause); the friendlier auto-fall-through-to-new is deferred pending
operator preference.

docs/contracts/issues/6.contract.md

@@ -372,3 +372,64 @@ test layer.
 - Issue #7 (mid-stream robustness, `MalformedSseData`) — landed; #6's
   pre/in-alt-screen split is orthogonal to #7's empty-data/malformed
   distinction (different error layers entirely).
+
+## Amendment 2026-06-30 — startup session picker (v1 coverage-audit, slice b2)
+
+The v1 coverage-audit found `list_sessions` had **no caller** — the startup
+session picker (design-brief §4: "single-session-per-launch, with a startup
+picker invoked when more than one session exists ... plus flags `--session`/
+`--new` to skip it") was never built. Bare TUI mode (neither `--session` nor
+`--new`) was a hard usage error. This adds the picker as a pre-alt-screen
+resolution step in `_resolve_then_run`, mirroring the existing `AgentPickerApp`.
+
+**Locked design (design-brief §4):** the picker is **resume-only** (§4 negative
+clause "no in-app session creation — `--new` flag only"); shown only when **>1**
+session exists (exactly 1 auto-resumes; the launch intent is "resume the last
+session I was poking at"). `--agent` stays a `--new` companion (forbidden in bare
+mode). **bare + 0 sessions → error** `[no_sessions]` directing the operator to
+`--new` (honors the "no in-app creation" clause; the friendlier
+auto-fall-through-to-new alternative is deferred pending operator confirmation).
+
+### `_parse` validation relaxation (ratatoskr.cli._parse)
+
+- Bare TUI mode (`send is None` AND no `--session` AND no `--new`) is now VALID
+  → triggers the picker. (Previously `raise UsageError("pass exactly one of
+  --session or --new")` unconditionally.)
+- `--send` mode still requires exactly one of `--session`/`--new` (non-
+  interactive: no picker can open) → `UsageError("--send requires --session or
+  --new")`.
+- `--session` + `--new` stays mutually exclusive.
+- `--agent` in bare mode → `UsageError` (`--agent` belongs to `--new`).
+
+```contract
+FN SessionPickerApp.__init__(self, sessions: list[SessionInfo]) -> None
+BRIEF: Textual App[str | None] startup session picker (mirrors AgentPickerApp, issue #8). Opens before RatatoskrApp when bare TUI mode resolves >1 session. `run_async()` returns the chosen session_id (str) or None on Esc/Ctrl-D/Ctrl-C dismissal. Architecturally separate from RatatoskrApp (list_sessions failures + dismissal land before any alt-screen — preserves #6 INV-001).
+PRE: [PRE-001 hard] sessions is non-empty -- assert sessions (caller resolves 0-session and 1-session cases BEFORE constructing the picker)
+POST: [POST-001 return_value] run_async() returns sessions[i].session_id for the highlighted row on `pick`, or None on dismiss -- assert result in {s.session_id for s in sessions} | {None}
+STEPS:
+  1. [setup, prescriptive] Store sessions; register the Australis theme (mirror AgentPickerApp).
+  2. [sequential, prescriptive] compose: Header + prompt Static + ListView of one ListItem per session (id-short + agent_id + last_active/name lines) + Footer.
+  3. [sequential, prescriptive] BINDINGS: enter→action_pick, escape/ctrl+d/ctrl+c→action_dismiss.
+  4. [branch, prescriptive] action_pick: read ListView.index; if None return (nothing highlighted); else exit(sessions[index].session_id). action_dismiss: exit(None).
+TESTS:
+  pick_returns_session_id [happy,tracer]: SessionPickerApp([s0, s1]); pilot highlights row 1 + press enter → run_async() returns s1.session_id.
+  dismiss_returns_none [happy]: press escape → run_async() returns None.
+  ctrl_d_dismisses [adversarial]: press ctrl+d → None.
+
+FN _resolve_then_run(args) — bare-mode extension (session picker)
+BRIEF: Before the existing new/resume branches, resolve bare TUI mode (not args.new AND args.session_id is None) via list_sessions + the picker. Sets a local `effective_new` and `resolved_session_id`; the existing branches then run unchanged on those locals.
+STEPS (inserted at the top of the `async with client` block):
+  1. [setup, prescriptive] SET effective_new = args.new; resolved_session_id = args.session_id.
+  2. [branch, prescriptive] IF (not args.new) AND (args.session_id is None):  # bare mode
+       a. CALL list_sessions(client) → page; ON SessionApiFailed → stderr `[session_api_failed]` + return 20; ON network error → `[network_error]` + return 21.
+       b. IF not page.items: stderr `[no_sessions] no sessions to resume; launch with --new --agent <id>` + return 14.
+       c. ELIF len(page.items) == 1: SET resolved_session_id = page.items[0].session_id.  # §4: picker only when >1
+       d. ELSE: SET resolved_session_id = await SessionPickerApp(page.items).run_async(); IF None → return 0 (Esc/Ctrl-D clean exit).
+  3. [sequential, prescriptive] Replace the two `if args.new` predicates with `if effective_new`; the resume `else` branch asserts + uses `resolved_session_id`.
+TESTS (in the `_resolve_then_run` block):
+  bare_zero_sessions_errors [error]: bare args; list_sessions → 0 items → stderr contains `[no_sessions]`; return 14; NO POST /sessions, NO picker.
+  bare_one_session_auto_resumes [scenario]: bare args; list_sessions → 1 item (sid="s-solo") → RatatoskrApp constructed with session_id="s-solo"; NO picker shown.
+  bare_multi_opens_picker [scenario,tracer]: bare args; list_sessions → 2 items; picker returns items[1].session_id → RatatoskrApp constructed with that session_id.
+  bare_picker_dismiss_exits_zero [scenario]: bare args; 2 items; picker returns None → return 0; RatatoskrApp NOT constructed.
+  bare_list_sessions_api_failure [error]: bare args; list_sessions raises SessionApiFailed(500) → stderr `[session_api_failed]`; return 20.
+```