chore: standardize renovate.json config

[tim-thacker-nullify]

Summary

• Standardize renovate.json to match the reference config from rebel-base
• Use config:recommended instead of config:base
• Add rebaseWhen: conflicted and standard schedule * * 1 * *
• Add standard packageRules: separate major updates handling for gomod, docker, GitHub Actions, Python
• Preserved repo-specific separateMajorMinor: false and regexManagers for golangci-lint

Test plan

• Verify Renovate bot picks up the new config on next scheduled run

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