chore: update checkout action to v7

[s1korrrr]

Summary

• pin actions/checkout 7.0.1 by full commit SHA in release and full-history secret-scan workflows
• retain least-privilege checkout configuration and full-history scan behavior
• update the source-contract test to enforce the new immutable pin

Verification

• ./script/check_open_source_readiness.sh
• swift package show-dependencies --format json
• swift test -Xswiftc -warnings-as-errors — 679 tests, 0 failures
• swift build -c release -Xswiftc -warnings-as-errors
• plutil -lint Resources/PrivacyInfo.xcprivacy config/*.entitlements
• ./script/build_and_run.sh --verify
• ./script/sandbox_smoke.sh — universal arm64/x86_64 bundle validated

Release note

This is repository maintenance for the latest release rebuild. It does not change DevScope product behavior.

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