Thread lifecycle: make Settled a real state and remove auto-archive

[Tryanks]

Summary

Make Settled a real lifecycle state, following T3 Code's design, and remove auto-archive. Settled becomes the only automatic lifecycle transition; archive is always an explicit user action.

Why

Today Settled (#408) has almost no effect:

• It is manual only (settle_session / make_session_active, crates/runtime/src/app/sessions.rs).
• Its only behavioural effect runs the wrong way: settled threads are exempt from auto-archive (auto_archive_candidates, crates/core/src/project.rs) and from orchestrate archive_on_complete, so finished work lives longer than untouched work.
• settled_at is only a cascade-undo key; nothing sorts by it and the Settled group has no paging.

Auto-archive is removed because archive is becoming destructive in part (it will move logs, call provider-native archive and clean up worktrees; see #529 and #530). The rule is: automatic transitions must be losslessly reversible. Settle is; archive no longer is. Neither T3 Code nor the Codex app auto-archives threads. Sidebar length is handled by the Settled shelf; disk growth by storage compaction and cold compression, independent of archive.

Spec

State. SessionMeta gains settled_override: Option<Settled | KeepActive> (replacing the bare settled_at flag), settled_at, unsettled_at. KeepActive is the user's "keep active" choice and suppresses auto-settle. Snooze (#527) and pinned (#528) are separate issues.

Last activity = the latest of: last user message, and the latest turn's requested / started / completed times. A thread that never ran never inactivity-settles.

Auto-settle when last activity is strictly older than N days. The auto settled_at is the activity time, not now.

Never auto-settle a thread that: is archived; has any override; has pending approvals or user input; has a starting or running session; has background work; has a user message still inside a 2-minute queued-turn grace window; is snoozed (unless it woke on an error or completion after the snooze). Linked-PR rules are in #534.

Automatic un-settle (resets any override, KeepActive included, to none):

• the user sends a message (also clears snooze);
• the session moves to starting or running;
• an approval or user-input request arrives.

Nothing else wakes a settled thread.

Settle stops the provider session, as in T3 Code. A thread idle for days has no warm prompt cache left (see #531), so stopping costs nothing and frees memory. The stop is dropped if the thread was re-engaged in the meantime. Manual settle keeps refusing while the thread family is busy.

Host sweep. The host evaluates auto-settle every 60 s, and immediately on settings changes and turn ends. It must not depend on a client: today auto-archive only runs when a desktop sidebar triggers it (crates/ui/src/sidebar.rs), which violates Principle 2.

Settings. auto_settle_after_days: Option<u32> (default 3, range 1–90, None = never), per-project overrides stored in host settings keyed by project id. Changing them does not reopen already-settled threads.

Sidebar.

• Section order: pinned, active, snoozed, settled.
• Settled shelf starts collapsed; shows 10 rows, then 25 more per "Show more"; sorted by settled_at desc, then last activity, then updated_at.
• Reopened threads enter the top of the active list via unsettled_at; later activity does not reorder the active list.
• The partition and sort live in core so desktop, phone and browser share them.

Undo. Settle shows a 5-second toast with Undo (and mod+z when no text field or terminal has focus). Undo is a client-sent reverse command; a claim token stops a stale toast from reversing a newer action. The same toast serves snooze, unpin and archive.

Protocol. SettleSessions, UnsettleSessions (keep-active), settle-related SettingsPatch fields; events carry the reason (user vs activity).

Removing auto-archive

• Remove auto_archive_disabled, auto_archive_max_idle_days, auto_archive_keep_count (crates/core/src/settings.rs), auto_archive_candidates, Command::AutoArchiveSweep and the sidebar-triggered sweeps.
• Threads already auto-archived stay archived.
• Open question: orchestrate archive_on_complete is an explicit per-orchestration choice (comparable to the Codex app archiving automation threads on completion). Keep it, but settled should no longer exempt a child from it.

Related work

• Add support for settling threads #408 introduced the current manual settle.
• guivieiras/tcode has unmerged work on local/integration touching the same surface: last_user_message_at and a "last user message" thread sort (45e74fc), thread state presentation and completed markers (874fefd, 9cd8805), and "settle actions" refinements (cf68c7e). It also replaces the unread dot with a "Mark completed" marker, which changes what the "unread" exemption means here. 874fefd rewrites ~1.2k lines of crates/ui/src/sidebar.rs into a sidebar/thread_row/ module, so the sidebar parts of this issue, Thread lifecycle: snooze #527 and Thread lifecycle: pinned threads #528 should be sequenced with it. The last-activity definition above should reuse last_user_message_at if that lands.

Reference

T3 Code origin/main f5ef0ddb9: apps/server/src/orchestration/ThreadSettlementPolicy.ts, ThreadSettlementReactor.ts, decider.ts (settle/unsettle), packages/contracts/src/settings.ts, docs/user/thread-sidebar.md; history in #4026, #4243, #8600, #9254, #12848.