test: cover Python and Go source normalization

[giri256]

What changed

Adds credential-free source-normalization coverage for Python and Go fixtures. The test also confirms Markdown files are ignored, path inputs are unchanged, and Python stdin input is marked as changed.

Why

Closes #21. The repository supports several source languages, but the checked-in fixtures only exercised TypeScript behavior.

Verification

• npm run check
• npm pack --dry-run
• node --test test/sources.test.mjs
• No public CLI or Action behavior changed
• No secrets, generated snapshots, dependencies, or model output included

[EmersonBraun]

Code review

No issues found. The contribution matches issue #21: it adds the requested Python and Go fixtures, keeps Markdown out of targets, verifies complete contents and language detection, covers path versus stdin isChanged, and avoids production-code or dependency changes.

[EmersonBraun]

LGTM

[EmersonBraun]

Thanks for this — the Python/Go coverage is a real gap this closes. Before merging, though: I applied this patch on top of current `main` to check it still passes (this branch is a few weeks behind), and one assertion now fails:

```
local source directories normalize supported files and ignore Markdown
AssertionError: expected ['example.go', 'example.py']
actual: ['README.md', 'example.go', 'example.py']
```

This isn't staleness in the usual "just rebase" sense — `.md` was added to the supported extensions list in `agents/code-review/sources.ts` since this PR was opened (Markdown is now a reviewable target, to support product-doc review). So `test/fixtures/sources/README.md` is genuinely picked up now, and the "ignore Markdown" premise in the test needs to change.

Two ways to fix it, your call:

• Update the assertion to expect all three files (`README.md`, `example.go`, `example.py`) and rename the test/comment away from "ignore Markdown", since that's no longer the behavior being covered — or
• If you'd still like an explicitly-ignored fixture in this test, rename `README.md` to something outside the supported-extension list (e.g. a `.txt` note) so the "ignored file" assertion stays meaningful.

Could you push either fix and rebase onto `main`? Happy to re-review right away once it's up — this is otherwise ready to merge (the Go/Python normalization coverage itself is correct and needed).