* docs: add manual testing guide for slash command validation
Adds a top-level TESTING.md that describes the manual test process PR
submitters must follow when their changes affect slash commands.
Includes:
- Process overview (identify affected commands, setup, run, report)
- Local setup instructions using editable install
- Reporting template for PR submissions
- Agent prompt that analyzes changed files and determines which
commands need testing, including transitive script dependencies
and extension hook mappings
* Update TESTING.md
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
* Update TESTING.md
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
---------
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
