Serve `/raw/` on the Hub fake and list an issue's comments on the GitHub one

Two endpoints the vendors have and the fakes did not. Neither is reachable
from a mirage client -- there is no `hf` verb that fetches a raw URL and no
`gh` verb that lists comments -- so both are covered in the selftests, which
is where this repo puts HTTP surface the battery cannot see.

`/raw/` is `resolve` for a reader rather than a downloader: `text/plain`, no
redirect, so a plain `requests.get` gets the file. It is how the Hub's own
web view links a README and how anything that never learned about LFS reads
one. The part that is not a synonym is LFS: a tracked path answers with its
POINTER, three lines naming the sha256 and the size. Probed rather than
recalled -- `/datasets/lockon/ToolACE/raw/main/data.json` is 133 bytes of
pointer against 37MB through `resolve`.

`GET /repos/:owner/:repo/issues/:number/comments` is oldest-first, which is
the whole of what it is read for: `comments[-1]` is only "what was said
last" in that order, and that is how a grader asks whether the reply it
wanted is the reply that landed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>