Extract normalizeWindowsDriveLetter to shared pathUtils module
Normalize workspaceFolder to uppercase drive letter to match OpenCode
server's path normalization. This fixes session visibility issues
where VS Code extension sessions don't appear in CLI/desktop and vice
versa due to case-sensitive directory comparison on Windows.
Changes:
- Create pathUtils.ts with shared normalizeWindowsDriveLetter function
- Update opencode.ts to import from pathUtils instead of inline definition
- Update ChatViewProvider.ts, AgentManagerPanelProvider.ts,
SessionEditorPanelProvider.ts to import from pathUtils
