Agent Tarmac v0.4.2 — startup that actually starts

Fixes two v0.4.1 startup regressions that came out of the non-blocking-startup rework.

Fixed

• The whole window froze on launch (no splash, blank sidebar). The token-stats compute was running on the UI thread, so its one-time cold read of your transcripts (~33s on a large machine) locked up the entire app until it finished. It now runs off the main thread — the splash plays, the sidebar loads, and the window stays responsive while the token count fills in.
• The sidebar could hang indefinitely. On a warm cache the startup scan finished so fast it emitted its results before the window had subscribed, so the sidebar sat on its loader until something else (a new terminal session) nudged it. The app now backfills the current session list on launch, so the sidebar populates on its own every time.

No change to what the app reads or sends — Agent Tarmac still only reads local transcript metadata and never uses your Claude usage.