fix(core): skip file watcher on filesystem roots

Description

The file watcher attempts to watch filesystem roots (e.g., C:/ on Windows, / on Linux) which causes unnecessary resource usage and potential permission issues.

Reproduction Steps

1. Open a project at filesystem root
2. File watcher attempts to watch the entire root directory
3. Causes high resource usage / permission errors

Expected Behavior

File watcher should skip filesystem roots and system directories.

Actual Behavior

File watcher attempts to watch roots, wasting resources.

System Info

• OS: Linux/macOS/Windows
• opencode version: dev

Fix

Added isRootOrSystemDirectory check in packages/core/src/filesystem/watcher.ts to skip roots (C:/, /, etc.)