Sessions/worktrees silently disappear from sidebar; cache never reconciled after cold start

[ymajoros]

Summary

Sessions (and entire worktrees) silently disappear from the sidebar even though their transcripts are intact on disk. They don't come back by reopening or restarting — the sidebar is rebuilt purely from the SQLite cache, and the cache never gets re-reconciled against the filesystem once it's populated.

Steps to reproduce

1. Use Switchboard for a while so several projects/sessions are indexed.
2. Quit Switchboard.
3. While it's closed, have sessions change or new ones appear (normal CLI use, new worktrees, etc.). Or simply update Switchboard to a build whose indexing differs from the one that first populated the cache.
4. Reopen Switchboard.

Expected: the sidebar reflects what's on disk under ~/.claude/projects/.
Actual: folders that weren't already in the cache show up empty (or not at all). Clicking them does nothing — there's nothing cached to show. The .jsonl transcripts are all still on disk.

Root cause

get-projects only triggers a filesystem scan when the cache is completely empty:

const needsPopulate = !isCachePopulated() || !isSearchIndexPopulated();
if (needsPopulate) {
  populateCacheViaWorker();   // full scan, cold start only
  return [];
}
return buildProjectsFromCache(showArchived);   // otherwise: cache only, never reconciled

buildProjectsFromCache reads strictly from session_cache. After the first cold start the cache is non-empty forever, so:

• folders that changed while the app was closed are not re-scanned (the live fs.watch only covers changes while running), and
• folders that were never indexed by the current build (their cache_meta.indexMtimeMs is 0) stay invisible permanently.

populateCacheFromFilesystem() exists for exactly this kind of full reconcile but is dead code — defined and exported, never called.

Impact

Common and confusing: sessions appear "lost" after an app update or a normal quit/relaunch cycle, with no UI way to recover them (reopen/restart don't help). The only workaround is forcing a re-index (e.g. touch-ing the .jsonl to trip the watcher, or wiping the cache so the cold-start scan runs).

Proposed fix

Reconcile the cache against the filesystem on get-projects, gated by mtime so it's cheap when nothing changed: re-index only folders that are new or whose newest .jsonl is newer than cache_meta.indexMtimeMs. PR incoming.

Environment: Linux, Switchboard 0.0.30 (reproducible on current main).