Add scenario framework and compatibility alignment

[davidahmann]

Summary

• add .tool-versions and CODEOWNERS
• add pinned tool-versioned lint-full and scenario test targets
• add Proof scenario fixtures and scenario test runner
• add Gait compatibility job to main CI

Validation

• make lint-full
• make test-scenarios
• go test ./...
• make prepush-full
• gait replace build/test against proof HEAD

[chatgpt-codex-connector]

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