Add S3-compatible artifact store

[JE-Chen]

Dependency-gated batch (approved under 批准全部). Full layers + tests + EN/Zh v41 docs + README.

Feature (utils/artifact_store, optional boto3)

• S3ArtifactStore: upload/download/list/delete reports, screenshots, and recordings against any S3-compatible bucket (AWS S3, MinIO, R2). The whole API is relative to the store prefix — upload returns a store-relative key that download/delete/url accept unchanged, and list strips the prefix back off.
• Injectable client + optional [s3]/boto3: the S3 client is a constructor arg, so the store's logic and the executor path are fully unit-tested with a fake in-memory client (no boto3/network). boto3 is imported only when no client is supplied; the live AWS path is honestly noted as CI-unverifiable. A module-level default store (configure_default_store) backs AC_s3_upload/AC_s3_download/AC_s3_list/AC_s3_delete (+ ac_*, Builder under Tools).

Verification

• 8 tests pass (upload relative-key + prefixed url, explicit key, round-trip download, list+delete, unconfigured-default raises, executor round-trip with fake client, wiring, facade); ruff clean; radon no CC≥C; bandit clean; PySide6-free.
• Stacked on fuzzy (Add fuzzy string matching and dedupe #248); rebased cleanly onto dev.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 53 complexity · 0 duplication

Metric	Results
Complexity	53
Duplication	0

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[sonarqubecloud]

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