docs(P00): clarify architectural decisions

Record 5 PO-approved architectural decisions (all confidence >=0.8):
- D-001: OpenAI-compatible BYOM contract for M1 (pluggable iface for M3)
- D-002: Relay Agent in Go (single static binary for curl|bash)
- D-003: AWS Secrets Manager (prod) + local-encrypted (dev) behind interface
- D-004: Postgres append-only + hash-chain audit for M1 (S3 WORM in M3)
- D-005: Next.js App Router + TypeScript single SPA

No escalations. Pipeline proceeds to RESEARCH.

---ci---
phase: 0
milestone: v0.1
status: clarify
---/ci---

.ciagent/CHECKPOINT.json

@@ -1,8 +1,8 @@
 {
   "phase": 0,
-  "stage": "specify",
+  "stage": "clarify",
   "milestone": "v0.1",
   "phase_role": "pre_execution",
   "attempts": 1,
-  "updated_at": "2026-08-24T22:40:00Z"
+  "updated_at": "2026-08-24T22:45:00Z"
 }