feat: add --follow-symlinks option for collection indexing

[moralesl]

Problem

qmd collection add and qmd update do not traverse symbolic links when scanning for files. Collections whose root directory contains symlinked subdirectories silently skip all files behind those symlinks.

Fixes #423.

Solution

Add a per-collection follow_symlinks option that controls fast-glob's followSymbolicLinks behavior. The setting is:

• Set via CLI: qmd collection add ~/vault --follow-symlinks
• Persisted in index.yml per collection:

  collections:
    vault:
      path: ~/vault
      follow_symlinks: true

• Respected by both collection add (initial index) and qmd update (re-index)
• Defaults to false (backward compatible)

Changes

File	Change
src/collections.ts	Added follow_symlinks to Collection interface, addCollection(), and updateCollectionSettings()
src/store.ts	Added followSymlinks option to reindexCollection(), passes to fast-glob
src/cli/qmd.ts	Added --follow-symlinks CLI flag, threaded through collectionAdd() → indexFiles(), updated help text

Testing

Verified manually:

• Without --follow-symlinks: symlinked directory skipped (3 files indexed)
• With --follow-symlinks: symlinked directory traversed (4 files indexed)
• qmd update respects the persisted follow_symlinks: true config
• All existing tests pass