Add independent production readiness acceptance gate

[nonsense]

Summary

• add a production readiness review between Sepolia/governance approval and mainnet
• require a two-key sign-off from the feature DRI and engineers outside the primary implementation group
• make the production-owning team explicitly accept normal on-call ownership
• require durable readiness evidence, invalidate sign-offs after material changes, and define a shared post-release monitoring period

Motivation

A feature should not be approved for production solely by the team that implemented it. Independent engineering acceptance makes operational knowledge, incident readiness, and ownership explicit before activation rather than transferring them after an incident.

Testing

• git diff --check
• npx --yes markdownlint-cli2 src/sdlc.md (reports 27 existing violations; origin/main reports 28)
• just test could not run locally because markdownlint-cli2 is not installed by the repository

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