Support symlinked workspace entries

[joelbqz]

Summary:

• Show symlinked markdown files and folders in the sidebar/search index using visible workspace paths.
• Preserve symlinks on save by writing through to the resolved target.
• Translate watcher events from symlink targets back to link paths, including symlinks created after workspace open.

Validation:

• vp check (existing E2E warnings only)
• vp test
• cd apps/desktop/src-tauri && cargo test
• cd apps/desktop/src-tauri && cargo clippy (existing warnings only)
• cd apps/desktop/src-tauri && cargo fmt --check