docs: add repository agent instructions

[mellowcroc]

Summary

• add root-level agent guidance for the MinaGuard monorepo
• define the online/offline signing compatibility boundary and cross-package update requirements
• require relevant audit-guide and README updates alongside code changes
• document security, validation, generated-file, and operational safeguards

Validation

• git diff --check
• verified all referenced repository paths
• verified every listed canonical check maps to an existing package script

Documentation-only change; no runtime tests were needed.