Support markdown symlinks in workspaces

[joelbqz]

Summary

• add symlink-aware Rust path classification shared by directory reads, indexing, writes, and watcher normalization
• include live markdown file symlinks and symlinked markdown directories in the sidebar/search index while skipping broken or recursive links
• preserve symlinks on save by writing to canonical targets, and map target watcher events back to logical workspace paths
• add a compact sidebar symlink badge plus specs/changelog/task updates

Validation

• vp check (passes with 2 pre-existing E2E lint warnings)
• vp test (27 files, 438 tests)
• cargo fmt --check
• cargo test (119 tests)
• cargo clippy (passes with pre-existing config/search/images warnings)