Docs: mention compare API in examples guide

[aaronlab]

Goal

Add one short note to examples/README.md so example readers know the compare result is also available through the local API while the BrowserTrace UI is running.

Context: the examples guide already shows browsertrace compare <failed_run_id> <success_run_id> and --json. It should also mention that local scripts or dashboards can call:

GET /api/compare/<failed_run_id>/<success_run_id>

Scope

Please keep this small:

• Update examples/README.md near the existing compare output / --json explanation.
• Add one sentence saying the local compare API is useful for scripts, dashboards, or automation checks that need the same first-divergence JSON payload while the local UI is running.
• Update tests/test_metadata.py so the examples README metadata test covers the new sentence.

Out of scope

• Do not change server behavior.
• Do not add new examples or generated output files.
• Do not edit unrelated docs.

Verification

Please run:

uv run --python 3.11 --extra dev pytest tests/test_metadata.py -q
git diff --check

Comment here before starting so I can mark the issue as claimed.

[kaizeenn]

I can take this one now. I’ll open a focused docs+metadata PR shortly.