[codex] Remove Digest and refactor feed and agent services

[versun]

What changed

• removed the Digest feature end to end, including model, admin entry, task/command paths, cron config, and related docs
• moved feed, agent, admin batch, and OPML logic into dedicated service modules so entrypoints stay thin
• added focused tests and updated architecture/refactor documentation to match the new layout

Why

• feed and agent behavior had grown across large model and view files, which made changes harder to reason about
• Digest had become extra surface area to maintain without fitting the current product direction

Impact

• feed updates, filtering, rendering, admin actions, and agent execution now live in clearer module boundaries
• Digest is no longer part of the product surface
• the repository docs now describe the current structure instead of the older layout

Root cause

• business logic had accumulated in broad entrypoint files and legacy Digest code remained alongside newer service-oriented paths

Validation

• uv run python manage.py migrate && uv run python manage.py collectstatic --noinput && uv run python manage.py test
• 671 tests passed