Add opt-in interactive-session idle auto-close (sessions.interactive_archive_after_hours)

[serxa]

Motivation — why you'd enable this

Nerve only cleaned up interactive sessions via a 30-day staleness backstop and the max_sessions overflow cap. Leaving idle chat sessions (web/telegram/…) open for days has real downsides that an opt-in short idle cutoff addresses:

• Timely memory consolidation. A session's content is memorized when it closes. A chat left open for days isn't consolidated into long-term memory until it finally does — a short idle cutoff makes that happen promptly, so the assistant's memory reflects recent conversations sooner.
• Context & cost hygiene. An idle session keeps its entire conversation context. Resuming a day-old thread reloads (and re-caches) all of it — well past the prompt-cache TTL, so it's a cache miss anyway. Closing idle sessions keeps active context focused instead of silently ballooning.
• Fresh starts. On chat surfaces it's usually desirable that a message sent after a long gap begins a fresh conversation rather than silently resuming yesterday's stale thread.
• No dangling questions. If the agent asked something via ask_user and the user never answered, that pending question lingers on the open session. Auto-closing the idle session expires it cleanly, so there's no phantom pending question (and nothing for the periodic expiry pass to try to inject into a session the user has clearly moved on from).
• Session-list hygiene. Keeps the active list tidy without waiting a month for the 30-day backstop.

It's opt-in precisely because the opposite preference is also valid — users who want long-lived, always-resumable sessions simply leave it off (the default).

Why a separate knob (not just a smaller archive_after_days)

archive_after_days is global — it archives every session class (cron, persistent-cron which reuse a session across runs, and external/satellite sessions). Lowering it to close chats quickly would also prematurely archive those and break their continuity. Interactive and background sessions simply want different retention, so this is a distinct, interactive-scoped policy — hence the explicit name interactive_archive_after_hours rather than a finer-grained archive_after_days.

What it does

Adds sessions.interactive_archive_after_hours (default 0 = disabled). When set > 0, the periodic cleanup also archives interactive sessions (web/telegram/discord/slack/whatsapp) idle longer than that many hours — memorizing them and expiring any pending ask_user question. Cron and persistent sessions are excluded (they keep their own long rotation / the archive_after_days backstop).

• SessionsConfig.interactive_archive_after_hours (+ from_dict).
• SessionManager.run_cleanup(..., interactive_archive_after_hours=0) — the short cutoff, skipped entirely when 0.
• SessionStore.get_stale_interactive_sessions and NotificationStore.expire_pending_questions_for_session (both additive).
• Gateway cleanup loop passes the config through.

Default behaviour is unchanged

• interactive_archive_after_hours defaults to 0, and the new cutoff block is skipped entirely when 0.
• The cleanup cadence stays every 6 hours by default; it only tightens to hourly when the feature is enabled (finer resolution is only needed then).
• New DB methods are additive; the new archived_interactive stat is 0 when disabled.

Testing

Added TestArchiveAndCleanup cases: disabled (default) leaves idle interactive sessions untouched, enabled archives them, and cron sessions are excluded even when enabled. Full suite: 1550 passed, 2 skipped.