Focus queue: prioritize recently-finished agents above running ones (#333)

[germanescobar]

Closes #333.

What changed

The on-radar (focus queue) sidebar used to sort pinned sessions by focusPinnedAt ?? createdAt ascending — i.e. oldest-pin-first. That buried the agent that just stopped on its own — the one that actually needs attention — under long-idle sessions the user forgot to unpin.

Replace the single ascending sort with a partition + two sorts:

1. Finished (inactive) sessions — most recently finished first (lastActiveAt desc).
2. Running (active) sessions — oldest running first, so the most recently started running session lands at the very bottom (lastActiveAt asc).

From top to bottom:

1. most recently finished  (lastActiveAt desc)
2. ...
3. oldest finished still on radar
4. oldest still running    (lastActiveAt asc, oldest first)
5. ...
6. newest still running    (lastActiveAt asc → newest at the bottom)

Why this shape

• The radar-inclusion filter (Boolean(session.focusPinnedAt)) is unchanged, so manually unpinned sessions still don't appear — and userUnpinned === true is server-side only, never reaches the client filter, so it can't regress.
• focusDoneAt does not enter the sort: sessions marked done clear focusPinnedAt on the server (updateSessionFocus in server/lib/sessions.ts) and the sidebar filter removes them, so they never reach the queue.
• lastActiveAt already doubles as the "finished-at" timestamp; the server updates it at the locations called out in the issue (reply, agent stream close). No new field needed.
• The sort is extracted into a small pure helper sortFocusQueue(items) exported from client/src/components/sidebar.tsx so it can be unit-tested in isolation. The useMemo in Sidebar just builds the items and calls the helper.

Acceptance criteria

• Given a mix of finished and running sessions on the radar, finished sessions appear above all running sessions.
• Within the finished section, the most recently finished session is at the top; the least recently finished is at the bottom of that section.
• Within the running section, the most recently started running session is at the very bottom of the queue; older-running sessions stack above it.
• No regression for sessions where focusPinnedAt is unset (still hidden from radar) or where the user has manually unpinned (userUnpinned === true).
• Behavior is unchanged on the server side; only the client-side sort changes.

Validation

• New test file client/src/components/__tests__/sidebar-sort-focus-queue.test.tsx — 6/6 pass:
  • all-finished → newest-finished on top
  • all-running → newest-running at the bottom
  • mixed → finished block above running block, each internally sorted
  • ties on lastActiveAt → stable-sort fallback to array order (matches the issue's explicit allowance)
  • active-state transition → a session flipping from active to inactive in the same render moves from the bottom of the running pile to the top of the finished pile
  • immutability → input array not mutated
• Full client test suite: 223/223 pass (no regressions in sidebar-delete-loading, sidebar-help-link, settings-responsive, agents-section, focus-advance-toast, or the lib helpers).
• node --import tsx smoke-import of sidebar.tsx succeeds (both sortFocusQueue and Sidebar export correctly).

Out of scope

• No time-decay window — "recent" is bounded by manual unpin / mark-done, not by a clock, matching the issue's non-goal.
• The CLI counterpart (related Add a CLI command to list on-radar (focus-queue) sessions #322, list on-radar sessions) is not implemented yet; when it lands it should mirror this ordering.