BUILD-10801: Set S3 as the default backend for all repositories

[julien-carsique-sonarsource]

Summary

• Removes the visibility-based split (public→GitHub, private→S3) in the auto-detection step; all repositories now default to S3
• Adds a was-github-default output from the backend-detection step to identify public repos that previously used GitHub cache by default
• Extends migration mode detection: in addition to explicitly forced S3, public repos with auto-detected S3 also enable import by default (they had GitHub cache entries to migrate)
• Updates README accordingly: intro, Backend Selection, import-github-cache default description, and Migration Mode resolution order
• Adds test scenario 5 to test-cache-migration-gh2s3.yml: auto-detected S3 on a public repo triggers GitHub import via was-github-default=true (distinct from the backend-source=forced path in scenario 1)

Test plan

CI-covered:

• Auto-detected S3 (public repo) → GitHub import enabled (test-auto-public-import-enabled)
• Auto-detected S3, forced backend: s3 → migration enabled (test-s3-import-enabled)
• import-github-cache: false input → migration disabled (test-s3-import-disabled)
• CACHE_IMPORT_GITHUB=false env var → migration disabled (test-s3-import-disabled-via-env)
• S3 hit → GitHub import skipped (test-s3-hit-skips-github-import)
• backend: github input → GitHub backend (test-action.yml)

Manual — will be covered after merge, before release, via dogfooding on the dummy repositories:

• Public repo, no override → S3 backend, migration enabled
• Any repo, CACHE_BACKEND=github env var → GitHub backend

[sonar-review-alpha]

Summary

Changes the default cache backend from visibility-based (public→GitHub, private→S3) to S3 for all repositories. Public repos now auto-enable GitHub cache import by default (was-github-default output) to maintain warm cache hits during migration—they previously used GitHub cache, so they have entries to migrate. Adds test scenario validating this new auto-migration path for public repos.

What reviewers should know

Start in action.yml to see the core logic change (lines 81–85): all repos now default to S3, public repos just set WAS_GITHUB_DEFAULT=true instead of switching to GitHub. The import resolution logic (lines 152–156) shows how this output gates migration enablement—public repos now take the same path as explicitly forced S3. The README updates explain why public repos get auto-migration (they had GitHub cache entries), distinct from the forced S3 path. Scenario 5 in the test workflow demonstrates this: a public repo with no backend input should auto-detect S3 and import from GitHub—verify it hits both checks.

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[bwalsh434]

LGTM!!

[sonarqube-cloud-us]

SonarQube reviewer guide

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[sonarqubecloud]

SonarQube reviewer guide

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonar-review-alpha]

LGTM! ✅

Clean, focused change. The logic flows correctly across all backend-selection combinations (forced input, env var, auto-detected public, auto-detected private), the new was-github-default signal is properly scoped to the auto-detection branch only, and the test scenario correctly exercises the new code path distinct from the forced-S3 path.

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