chore(deps): update actions/checkout action to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
actions/checkout	action	major	v4 → v6
actions/checkout	action	major	v5 → v6

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Release Notes

actions/checkout (actions/checkout)

v6

Compare Source

v5

Compare Source

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Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about these updates again.

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• If you want to rebase/retry this PR, check this box

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[MisileLab]

/review

[SimplyCrystal]

Review Summary

✅ APPROVED - This is a safe dependency update.

Changes

• Updates actions/checkout from v4/v5 → v6 across 5 workflow files
• Affects: agent.yml, auto-review.yml, corin-docker-build.yml, gqldb/.github/workflows/ci.yml

Analysis

V6 Changes:

• Internal credential storage architecture change (from .git/config to separate file with includeIf)
• No breaking changes to public API or default behavior
• All inputs remain compatible (token, fetch-depth, etc.)

Compatibility Verified:

• ✅ Custom PAT tokens (token: ${{ secrets.GH_PAT }}) - fully supported
• ✅ Full history checkout (fetch-depth: 0) - fully supported
• ✅ Standard checkout without special options - fully supported

Recommendation

Safe to merge. The credential storage change is transparent to users and improves security isolation.