feat: add /debug-test slash command for analyzing test failures

[emmaaroche]

Description

This PR introduces a new /debug-test Claude Code slash command to help analyze test failures and suggest debugging steps and fixes. This was inspired by the recent increase of failures in the nightly pipeline.

While many failures are already familiar to debug, the command is intended as a lightweight guide, particularly useful when investigating failures in less familiar testing areas or when encountering new or uncommon issues, providing a quicker starting point for debugging.

Changes

• Adds a structured 4-step workflow:
  • Extract the failing file path from the traceback
  • Trace test fixtures and imports
  • Categorize the failure type
  • Provide suggested debugging steps and fixes
• Documents common failure patterns derived from an analysis of recent nightly test failure logs done by Claude
• Defines a response output format for the slash command

Verification steps

To verify the /debug-test slash command:

1. Check out this branch and confirm /debug-test is present in .claude/commands/
2. Initialize Claude and run /debug-test, followed by a test failure output
3. Verify that the command identifies failure patterns and suggests appropriate debugging steps

[silvi-t]

I'm not sure to what extent Claude will be able to correctly determine the root cause if it's not a simple error in the fixture or test design. In any case, he can at least provide some additional context and guidance. So I am okay with approving this PR.