research(federation): agent-to-agent coordination L3 — extends #172

[Argocyte]

Design L3 agent-coordination layer using sociocratic self-similarity. Three options: (α) S3 grammar over existing AP, (β) capability broadcasting via AP JSON-LD extensions, (γ) cross-cooperative attestation web-of-trust. Extends #172. UUID: 4ee7e456-7944-450e-94c0-af1ae8eac1f4. Filed by Et under G5/A7.