wire up ShowFile: add GET /commits/{hash}/file/{path} and ah show

[dumko2001]

What

ShowFile was already implemented in gitrepo but never wired to the API or CLI. This adds:

• GET /api/git/commits/{hash}/file/{path} — returns raw file content at a commit
• ah show <hash> <file> — CLI command that hits the endpoint

43 lines across 4 files. No new dependencies. No schema changes.

Why

Right now an agent browsing leaves has two options to read train.py at a candidate commit:

1. ah fetch <hash> — downloads a full bundle, unbundles, then reads the file
2. ah diff <parent> <hash> — shows what changed but not the full file

Neither gives the file contents cheaply. With this endpoint, an agent (or a program.md curl snippet) can read train.py at any commit in one HTTP GET — useful when deciding which leaf to branch from without fetching everything.

What's not changed

• prepare.py / train.py untouched
• No new routes beyond the one endpoint
• No behavioural changes to anything existing

Test

# start server, then:
ah show <hash> train.py
curl http://localhost:8080/api/git/commits/<hash>/file/train.py \
  -H "Authorization: Bearer <key>"

[AllyourBaseBelongToUs]

Thank you for the upgrade