docs: commit all Phase 6 documentation updates and OpenSpec archives

- devops docs: 8 files updated for Phase 6 state; field-trial.md added (946-line runbook)
- developer docs: api-reference (50+ endpoints), quick-start, 5 existing guides updated, 5 new guides added
- engineering docs: all 12 files updated (services, architecture, SDK guide, testing, overview)
- OpenSpec archives: phase-7-devops-field-trial, developer-docs-phase6-update, engineering-docs-phase6-update
- VALIDATOR.md + scripts/start-validator.sh: V&V Architect tooling added
- .gitignore: exclude session artifacts, build artifacts, and agent workspaces

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openspec/changes/archive/engineering-docs-phase6-update/.openspec.yaml

@@ -0,0 +1,23 @@
+id: engineering-docs-phase6-update
+title: "Engineering Documentation — Phase 6 Complete Update"
+status: complete
+proposed: 2026-04-04
+approved: 2026-04-04
+approved-by: CEO
+completed: 2026-04-04
+workstreams:
+  - id: WS1
+    title: "05-services.md — add Phase 3–6 service deep dives"
+    status: complete
+  - id: WS2
+    title: "02-architecture.md — update component diagram and data flows"
+    status: complete
+  - id: WS3
+    title: "11-sdk-guide.md — add Rust SDK section"
+    status: complete
+  - id: WS4
+    title: "09-testing.md — add AGNTCY conformance, tier, analytics test coverage"
+    status: complete
+  - id: WS5
+    title: "01-overview, 03-tech-stack, 04-codebase-structure, 06-walkthroughs, README — Phase 3–6 updates"
+    status: complete