fix: make production runbook execution deterministic

[areshand]

Summary

• add a generic ops bind-runbook command that writes the canonical Markdown path and exact prefixed content digest
• require reviewers to reject failed bindings and execution agents to use the exact reviewed file
• route external-service goals through discovered Markdown runbooks instead of provider-specific model tools or implementation workflows

Validation

• cargo test certified_runbook_digest_uses_prefixed_exact_bytes
• cargo test prod_ops::tests

E2E blocker fixed

The Slack E2E generated a request without runbookDocument/runbookContentSha256, then attempted a manual review and reconstructed execution request. This patch makes the certified binding an explicit generic CLI step before independent review.