Server overwhelmed by concurrent project bootstraps when many sidebar projects exist

[BYK]

Description

When the web UI loads with many sidebar projects (e.g. 17 directories under ~/Code), all projects bootstrap concurrently. Each bootstrap spawns 5-7 subprocesses (git, ripgrep, parcel/watcher), resulting in ~100 simultaneous subprocesses that overwhelm the event loop and saturate I/O on modest hardware (4-core, SATA SSD).

Additionally, Filesystem.exists() wraps existsSync() in an async function — it looks async but blocks the event loop on every call. This prevents the server from responding to health checks and API requests while directory walk-ups are in progress.

Steps to reproduce

1. Configure OpenCode with 15+ sidebar projects
2. Start opencode serve
3. Open the web UI
4. Observe slow/unresponsive UI — health checks timeout, API routes are delayed

OpenCode version

1.2.26

OS

Linux (also affects macOS, Windows)