feat(update): add -c/--collection filter to qmd update

[erlebach]

fix(update): add -c/--collection filter to qmd update

Summary

• qmd update now accepts -c <name> (or --collection <name>) to re-index a single collection instead of all collections
• Multiple -c flags are supported (e.g. qmd update -c notes -c work)
• The completion message now names the collection when only one was updated (e.g. ✓ 'notes' updated. vs ✓ 3 collections updated.)
• Fix: added transaction to the Database interface in src/db.ts, which was used in store.ts but missing from the type definition (caused a build failure under updated better-sqlite3 type declarations)

Behaviour

qmd update                    # unchanged — updates all collections
qmd update -c mynotes         # updates only the 'mynotes' collection
qmd update -c notes -c work   # updates two specific collections
qmd update -c nonexistent     # exits with error: "Collection 'nonexistent' not found."

Files changed

• src/cli/qmd.ts — updateCollections() accepts optional collectionFilter?: string[]; filters and validates before iterating; updated completion message
• src/db.ts — added transaction<T> to the Database interface