docs: unify vignette around query_files(), drop search_files() discussion

Replace all search_files() examples with query_files() equivalents.
Remove the "search_files() Still Works" section and the dual-API framing.
Add examples for glob matching, extension/datatype filters, tibble output,
derivative_files() wrapper, and participants(as_tibble = TRUE).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bbuchsbaum