docs: add architecture decision records (ADRs)

[imran-siddique]

Create docs/adr/ directory with ADR template and 3-5 ADRs documenting key architecture decisions: (1) Why Ed25519 over RSA for agent identity, (2) Why 4 execution rings instead of RBAC, (3) Why IATP handshake has 200ms SLA, (4) Why policy engine is deterministic (no LLM). Use the MADR template format (https://adr.github.io/madr/). Each ADR should be 100-200 words with Context, Decision, Consequences.