Add a partial-GitHub-outage fault matrix for zed-cli source fallback #190

Independent fault-injection follow-up for zed-pkg/zed-cli#388.

A total zpkg.net outage is now covered in the candidate repo. Add a test-org matrix that independently fails GitHub surfaces so source ordering and degradation behavior are proven rather than inferred.

Scenarios:

• GitHub REST API degraded while smart-HTTP/tag archive remains usable;
• Release metadata/asset unavailable while GHCR is usable;
• Release + GHCR unavailable while tag archive is usable;
• codeload/tag archive unavailable while Release asset is usable;
• GitHub sources unavailable with explicitly configured R2 mirror as last resort;
• all fallback sources unavailable -> bounded deterministic failure.

Acceptance criteria:

• pin candidate by immutable SHA;
• each scenario has fail-if-used sentinels for higher/lower-priority sources as appropriate;
• assert source selection and terminal error category;
• run with clean HOME/store and no ambient credentials unless the scenario explicitly tests auth;
• sanitized evidence only; no secrets/signed URLs.