chore: retire active v8-runner manifest

[zeegin]

Summary

• remove the active v8-runner manifest now that Ingvar Consulting maintains the fork and its release pipeline
• retain all previously published immutable toolchain releases as historical artifacts
• add a repository contract test preventing accidental reactivation

Verification

• python3.12 -m unittest discover -s tests
• python3.12 -m py_compile scripts/.py toolchain/.py toolchain/builders/.py tests/.py
• git diff --check