feat(session): persist AI-generated session titles

[maxnoller]

Summary

Generate a short title (3–7 words) via the auxiliary model after the first user→assistant exchange and persist it on the session row. The chat sidebar (/chat) and /api/chat/recent prefer the stored title, falling back to the existing first/last-message preview when none exists. Pattern mirrors Hermes Agent's agent/title_generator.py and DeerFlow's TitleMiddleware.

Why

Today the sidebar always derives a title on read from the first/last message via buildSessionBoundaryPreview (src/session/session-preview.ts) — clunky labels like "hey can you" ... "Sure — here's the…". A real, persisted title makes sessions scannable and searchable by topic.

What's in this PR

• Schema migration V20 (src/memory/db.ts): nullable title and title_source columns on sessions via addColumnIfMissing.
• DB helpers: setSessionTitle(id, title, source) (auto source uses WHERE title IS NULL for race-safety), getSessionTitle(id), countUserMessagesForSession(id). getRecentSessionsForUser prefers the stored title and falls back to the existing boundary preview.
• Title generator — new src/session/session-title.ts:
  • generateSessionTitle(...) — Hermes-style "3–7 words, return only title text" prompt; normalizer strips quotes, Title: prefix, <think> tags, trailing punctuation; caps at 80 chars; rejects empty/single-char/untitled outputs.
  • maybeAutoTitleSession(...) — sync gate (userMessageCount > 1 or existing title → bail), then void (async () => …)() fire-and-forget body wrapped in withSpan('hybridclaw.session.title', …). Never blocks the user-visible reply.
• Auxiliary task wiring: new 'session_title' task added to TASK_MODEL_KEYS and auxiliaryModels in runtime-config.ts + config.example.json (default provider: "auto", override with AUXILIARY_SESSION_TITLE_MODEL / _PROVIDER).
• Gateway hook (src/gateway/gateway-chat-service.ts): hooked into all three recordSuccessfulTurn sites — concierge respond, version-only, main success — using the just-recorded exchange (no re-fetch, race-safe vs compaction).
• Console: no changes — chat-sidebar.tsx:111 already renders s.title, which the gateway now resolves to stored-or-derived.

Out of scope (next iteration)

• /title slash command for manual override (the title_source column is reserved for 'user').
• Backfill for pre-feature sessions (they keep deriving from boundary preview — zero regression).
• Re-titling on long sessions / topic drift.
• Pushing fresh titles to the sidebar in real time (next refresh picks them up).

Test plan

• npm run typecheck (root + console)
• npm run check (biome)
• tests/session-title.test.ts — 12 new unit tests (normalizer, generator happy/error paths, gate behavior)
• tests/memory-service.test.ts — 5 new cases (set/get round-trip, race-safety with auto source, user override, countUserMessagesForSession, stored title surfaces through getRecentSessionsForUser)
• npm run test:unit (root): 2874 passing — the 9 remaining failures (admin-terminal, eval-command, host-runner.*) are pre-existing flakes confirmed on main baseline
• npm --workspace console run test: 216 / 216 passing
• Manual smoke: start gateway + console dev, send a first message in a new session, confirm sidebar updates from derived preview to a stored 3–7 word title within ~1s of reply; restart gateway and confirm the title persists.

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