feat(agricola): add Auto-native proposal workflow

[brendanjryan]

Motivation

Replace command-driven scheduled Agricola orchestration with an Auto-native proposal and approval flow.

Summary

• add a read-only scout that continuously discovers SDK porting work and creates deduplicated proposal issues
• add an approved-ticket implementer that opens draft Rust or Python PRs and handles natural issue, review, and CI feedback
• make the legacy Actions workflows manual rollback paths
• document deployment, approval, permissions, and rollback

Key design considerations

• GitHub issues and labels form the durable approval queue
• the scout cannot write SDK repositories; the implementer cannot merge or write secrets and workflows
• issue and PR bindings keep ordinary human feedback in the original implementation session

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