refactor(desktop): derive working/attention session sets from $sessionStates

[ethernet8023]

What does this PR do?

$workingSessionIds and $attentionSessionIds were independently-maintained
atoms that updateSessionState had to manually keep in sync with the session
cache — paired setSessionWorking/setSessionAttention calls on every state
tick, plus a rotation special-case in ensureSessionState. This is the classic
"projection atom drifted from the cache" bug class: two sources of truth that
must agree but nothing enforces agreement.

This PR makes them pure computed() projections of $sessionStates,
so the data flow is one-directional:

gateway event → cache → $sessionStates → computed views ($working, $attention)

All transition side-effects (watchdog arm/disarm, settle grace, unread
marker, compression id rotation signal) move into a single handleTransition
function fired from publishSessionState by diffing previous vs next — one
choke point instead of per-callsite bookkeeping.

The watchdog's force-clear (8 min of stream silence) now reaches the cache
through a setWatchdogClearFn callback the cache wires on mount, instead of
a listener set.

Related Issue

No issue — self-driven refactor found during review of the session-status
data flow.

Type of Change

• ♻️ Refactor (no behavior change)

Changes Made

• store/session-states.ts

  • $workingSessionIds / $attentionSessionIds are now computed($sessionStates) projections — deleted the independently-maintained atoms + their setters (setWorkingSessionIds, setAttentionSessionIds, setSessionWorking, setSessionAttention)
  • Watchdog, settle-grace, unread-finished, and compression id-rotation logic consolidated into handleTransition(prev, next, runtimeId), called from publishSessionState
  • clearAllSessionStates() now disarms all watchdog timers and drops settle-grace entries — a gateway switch can no longer leak stale 8-min timers or let the old gateway's sessions squat in the sidebar merge keep-set for 30s
  • dropSessionState() disarms the dropped runtime's watchdog timer
  • setWatchdogClearFn replaces onSessionWatchdogClear listener pattern

• store/session.ts — removed ~180 lines of now-dead watchdog/settle/attention/unread code that moved to session-states.ts

• app/session/hooks/use-session-state-cache.ts

  • updateSessionState no longer manually calls setSessionWorking/setSessionAttention/noteSessionActivity — publishSessionState handles all transitions
  • ensureSessionState creates a new state object on stored-id rotation (instead of mutating in place) so updateSessionState can detect the transition
  • Watchdog clear wired via setWatchdogClearFn effect

• Import sites — 8 files updated to import $workingSessionIds/$attentionSessionIds/getRecentlySettledSessionIds from session-states instead of session

How to Test

1. npm run typecheck — both tsconfigs clean
2. npm run lint — eslint clean
3. npm test — 214 files, 1792 passed / 1 skipped
4. Watchdog tests now exercise the real timer→callback wiring (via setWatchdogClearFn) instead of manually simulating the clear — and a new test asserts clearAllSessionStates disarms all timers

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run npm test and all tests pass
• I've added tests for my changes — watchdog tests rewritten to test transitions through publishSessionState (the real public surface), plus a new clearAllSessionStates disarm test
• I've tested on my platform: NixOS 25.05, Hyprland

Documentation & Housekeeping

• N/A — no config keys, no tool changes, no new skills