fix(core): skip file watcher on filesystem roots

[CannonRS]

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.)

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• watcher.ignore never reaches the embedded fff engine — per-dir inotify watches on every indexed dir (192K watches / 4.5GB RSS on a large root); root .ignore file is the only effective channel #37740: Covers the same root cause — file watcher registers inotify watches on every indexed directory when opened at a large/root path, causing high resource usage. Includes analysis of the fff engine behavior and a workaround.
• opencode serve pegs CPU at ~1000% — fff content-indexes entire $HOME (search.ts enableHomeDirScanning/FsRootScanning) #32511: fff content-indexes $HOME (a near-root path) causing ~1000% CPU usage — same class of problem.

If your fix is distinct from what's described there, please clarify the difference in your issue.