Wiring up session Archive (sidebar + Session History + auto-unarchive)

[yixuanzhong]

Following up on #10069 (sidebar delete) — a maintainer noted that archive is the primitive you'd rather lean on than delete, and that the data model already has an unused archived_at field. This is a proposal to actually wire archive up, per the contributing guide's "discuss direction first."

What already exists

• ACP methods _goose/unstable/session/archive and .../unarchive (ArchiveSessionRequest_unstable / UnarchiveSessionRequest_unstable) → on_archive_session / on_unarchive_session, which set/clear archived_at.
• Session.archived_at on the model, DB column, and it's returned in SessionInfo.

What's missing (why it's currently a no-op)

1. The session list doesn't filter on archived_at — list_sessions_matching has no archived clause, so archived sessions still show everywhere. Archiving hides nothing.
2. No UI — neither the sidebar "Chats" list nor Session History has an archive/unarchive affordance.
3. New activity doesn't clear archived_at — add_message only bumps updated_at, so an archived chat you keep talking in stays hidden.

Proposed direction

• Sidebar: an Archive action (non‑destructive, reversible) that removes a chat from the recent list — a gentler alternative to delete there.
• Session History: an Archive/Unarchive toggle per row + an "Archived" badge, so archived chats are discoverable and restorable. (Open question below on whether to also hide them by default behind a filter.)
• Backend:
  • Exclude archived sessions from the recent/sidebar list (e.g. archived_at IS NULL, with an include_archived opt‑in for an archived view).
  • Auto‑unarchive on new activity: clear archived_at when a message is appended (add_message), so continuing a conversation brings the chat back — Gmail‑style. (Only the live append path; bulk replace_conversation for import/replay would be left alone.)

Open questions for maintainers

1. Auto‑unarchive on new activity — desirable, or should archive be sticky until explicitly unarchived?
2. In Session History, prefer an "Archived" badge inline (archived still listed) or hide archived by default behind a filter/tab?
3. Should the sidebar per‑row action be archive only, or archive + delete?

I have a working implementation of all of the above and am happy to open a PR (or split it — the archived_at IS NULL list filter is the smallest self‑contained piece). Wanted to align on direction first.