fix: pass pre-read content to bypass fs.existsSync for WSL agent configs (#10451)

When VS Code runs on Windows host with a WSL remote workspace, selecting
an agent config from .continue/agents/ fails because localPathOrUriToPath
cannot resolve the vscode-remote:// URI to a local path, causing
fs.existsSync to return false and falling through to the deprecated JSON
config loader.

The fix passes pre-read content (already available in
overrideAssistantFile.content) through the PackageIdentifier, and checks
for it before the fs.existsSync gate. This reuses the content bypass
added to RegistryClient in PR #9739.

Fixes #10450

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>