docs(spec): define agent-native execution and credential hardening

[projectious]

Summary

• position ainfra as the agent-native infrastructure execution boundary with complete CLI parity
• define credential acquisition, actor separation, durable operations, evidence, and adversarial security
• make the standardized infrastructure result a producer-owned proof artifact
• keep Aibox consumption optional for local and independently configured targets
• commit the non-secret verifiable consumer target handover as planned work
• support opaque external correlation plus durable idempotency without creating a global identity model

Validation

• scripts/validate-v1-spec
• git diff --check

Review focus

Please review the agent/execution division, authorization and credential boundaries, product-owned result semantics, optional consumer handoff, and roadmap sequencing.