feat: authored first-message presets — auto-seed on session-create (v0.19.8)

Codifies 'give an agent a first message' (Worldtree #347): new module
ratatoskr.first_message (FIRST_MESSAGE_PRESETS + seed_preset_first_message)
seeds a preset agent's opening as a #347 authored turn-0 on every new session,
wired into all three create paths — cli._amain (--send --new), tui._resolve_then_run
(bare --new), web._create_session_endpoint (POST /api/sessions).

seed_preset_first_message is strictly best-effort (INV-001): it soft-guards its
inputs (return None, never assert), bounds the write with asyncio.wait_for so a
stalled /history can't block create (the CLI/TUI clients disable read timeout for
SSE), and swallows every exception except asyncio.CancelledError (which
propagates) — so it can NEVER raise into or block the session-create path it is
wired into. Per-content idempotency key → idempotent replay, no dup.

Seeded with ratatoskr:sindra, whose opening greeting moved out of her card:
her live system_prompt was PATCHed (non-destructive) to drop the Startup
workaround the #347 first-message now replaces.

Quality gate (both cross-frontier panels): heid-code-review returned zero
implementation drift (2 test-only fixups applied); heid-bug-hunt caught the
gap the conformance lens can't see — code matched the contract's narrow
ERROR_ROUTING but INV-001's 'never raises' is broader — driving the broad-except
+ soft-guard + wait_for hardening above.

Contract docs/contracts/first_message.contract.md (module-scoped, validated).
TDD: 12 unit + 1 web wire-in; the 3 existing sindra bind tests gained a
history-endpoint mock (creating a preset agent now auto-seeds). Suite 615 green,
ruff+mypy clean. Auto-seed live-proven generation-free against personal :8081.

tests/test_tui.py

@@ -2922,6 +2922,10 @@ class TestTuiBifrostBind:
                 },
             )
         )
+        # sindra is a preset agent → the TUI create path now auto-seeds a #347 first-message.
+        respx.post("https://w.example/sessions/s-bound/history").mock(
+            return_value=httpx.Response(201, json={})
+        )
 
         async def fake_run_async(self) -> int:
             return 0