feat(update): add -c flag to update specific collections

[JasonOA888]

Problem

The -c/--collection flag works with search commands but not with qmd update. Users with many collections have to update all of them even when they only want to re-index specific ones.

Solution

Add collection filtering support to updateCollections() function:

qmd update -c notes              # Update only notes collection
qmd update -c notes -c docs      # Update multiple specific collections  
qmd update                       # Update all (existing behavior)

Changes

• Add optional collectionFilter parameter to updateCollections()
• Filter collections list when -c flag is provided
• Show helpful message if no collections match the filter

Fixes #298