feat(cli): afm check diagnostics parity (--format) + document new codes

[P4suta]

Part of #81. Gated on the next aozora pin bump that crosses feat/devex-inner-loop. Lightweight / optional.

Background

The 12 new authoring diagnostics (aozora f810ee0 8 "clean" + 25dc003
4 model-dependent) already surface through afm with no code change:
render_to_string returns them in Rendered.diagnostics, and afm check
prints each generically as diagnostic [code]: …
(crates/afm-cli/src/main.rs). So this issue is polish, not a blocker.

Optional work

1. Parity with aozora's aozora check (6ccbc22, graphical + json/short
   output): add --format {human,json,short} to afm check in
   crates/afm-cli/src/main.rs; human stays the default
   (back-compatible).
2. Document the 12 diagnostic codes (coordinate with the docs issue).
3. Extend crates/afm-cli/tests/cli_integration.rs — assert at least one
   new code round-trips, and (if added) the --format json shape.

Decision to make

Whether afm-cli should own a --format flag at all, or defer diagnostic
formatting entirely to downstream consumers of the library
diagnostics. Resolve before implementing.

Re-confirm the diagnostic code strings at the pinned SHA.