test: add diagnostic fixtures

[CoderDeltaLAN]

Adds minimal diagnostic repository fixtures for future agent instruction checks.

Scope:

• add fixture repositories for empty, single-agent, multi-agent overlap, and risky instruction cases;
• add tests that verify fixture files exist, supported instruction path shapes are represented, and fixtures do not contain obvious raw secret patterns.

Not included:

• no diagnostic engine implementation;
• no CLI changes;
• no CI changes;
• no README final changes.