[FEATURE] Opt-in auto-archive for Routine-generated sessions after N idle days

Summary

Sessions generated by recurring scheduled Routines pile up un-archived indefinitely, because auto-archiving isn't something the user is expected to remember to do for automated runs the way they naturally do for finished feature work. Please add an opt-in setting to auto-archive Routine-generated sessions after N days of no further activity/reference.

Current behavior

Looking at my own session history: sessions from one-off feature work get manually archived once the work is done, but sessions generated by recurring "daily check" style Routines are almost never archived — they just accumulate in the active session list indefinitely, month after month, even though each one was a fully self-contained, completed run. There's no built-in mechanism to clean these up short of manually archiving each one, which nobody does for routine-generated noise.

Motivation

Routines are meant to run unattended — that's the whole point. But the sessions they leave behind still require attended cleanup, which nobody does in practice, so they just accumulate. This is purely a bookkeeping annoyance rather than a functional bug, but at the scale of months of daily/near-daily firings across several concurrent Routines, it adds up to a lot of dead weight in session lists and search results.

Proposed behavior

1. Opt-in, per-Routine or global setting: "auto-archive sessions generated by this Routine after N days if untouched."
2. Only applies to sessions the user never manually opened/interacted with beyond the automated run itself — a Routine session the user actually engaged with (replied, asked follow-ups) should NOT be silently auto-archived.
3. Archived, not deleted — fully reversible, same as manual archiving today.
4. Clearly surfaced as an explicit, user-configured setting — not a background behavior enabled by default.

Why it matters

• Reduces sidebar/search clutter proportional to how long-running and numerous a user's Routines are — the more valuable Routines become (longer history, more of them), the messier the session list gets without this.
• Keeps the "archive = I'm done with this" mental model consistent between manual work and automated work, rather than only applying to the former.

Related issues (adjacent, important caution)

• #58446 Sessions auto-archiving within 30-90 min (bug) — this is a report of unwanted, unexpected auto-archiving happening today as a bug. This request is explicitly the opposite: an intentional, user-opted-in, clearly-configured behavior. Any implementation of this feature needs to be trivially distinguishable from that bug — e.g. very visible settings UI, clear audit trail of what got auto-archived and why — so it doesn't get conflated with or contribute to that existing complaint.
• #79080 [FEATURE] Consolidated digest for multiple scheduled Routines firing close together — companion Routines-hygiene feature.

Notes

• Reviewed for duplicates before filing (searched "auto-archive idle sessions routine") — the only close match (#58446) is a bug about unintentional auto-archiving, not a feature request for opt-in cleanup, so this is a distinct ask — but given that existing bug, this should probably ship carefully and behind an explicit opt-in, not a default-on behavior.