DigestLevel gains two methods:
- gather(): returns (label, inputs) for a given arg — daily reads
journal entries, weekly/monthly compute child labels and load files
- find_args(): returns candidate args from journal dates for auto-
detection, handling per-level completeness checks
Public generate_daily/weekly/monthly become two-liners: gather + generate.
digest_auto collapses from three near-identical phases into a single
loop over LEVELS.
Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>