feat(completion): annotate the file-basename pickers with size and date

Eight completing-read pickers listed bare file basenames, so marginalia had no directory to resolve and couldn't annotate them. Add cj/completion-file-annotator to system-lib — an annotation-function factory that takes a candidate->path resolver and yields a size + modification-date suffix (or "dir" for directories, nil for missing files). Wire each picker through cj/completion-table-annotated with a per-site category and resolver: timer sounds, drill flashcards, Info files, the test-runner focus add/remove, vc clone dirs, hugo drafts, and agenda projects (the project's todo.org mtime). music-config's existing completion table gains the category and annotator inline, keeping its sort metadata.

The candidate strings and every return value are unchanged — this only adds completion metadata — so all downstream logic is untouched. The six modules that didn't already pull in system-lib now require it.

Tests: cj/completion-file-annotator gets Normal/Boundary/Error coverage (file, directory, nil path, missing file). Full suite green at 5394.

Claude-Session: https://claude.ai/code/session_014fyKMTTqLrZpL3rDF3dYc3

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