Add docs/swarm/review-classifier.md, the high-level architecture of the
planned PR review classifier pipeline, written as the full target shape
under a "planned, not implemented yet" banner. Link it from docs/README.md
and the docs/swarm/README.md piece list.
The page reflects the epic description and the operator's answers in the
epic thread; points still open are listed in its "Still open" section.
The banner names the epic by title rather than by number because the
tracker-tag lint rejects issue references in tracked files.
