targets.ini: move Repositories out of [DEFAULT] into a dedicated [github] section

[gaurav]

Background

tests/targets.ini currently stores Repositories = ... under [DEFAULT], which means it is treated as a target attribute and inherited by all target sections. Repositories are a GitHub configuration concern, not a per-environment target attribute.

Proposed fix

Move Repositories into a dedicated [github] section (or a separate github.ini / top-level config file), and update the code that reads it in tests/github_issues/conftest.py accordingly.

Notes

• Identified during the Opus code review of PR Add GitHub-issue-driven test framework, assertion handlers, and csv-to-babeltests CLI #67.
• Not blocking; the current approach is an improvement over the previous hard-coded value in conftest.py.
• Affects tests/targets.ini and tests/github_issues/conftest.py.