feat(FN-019): draft enhanced architecture.md with all 18 required sections (+4 more)

Commits merged:
- docs(FN-019): complete Step 6 — documentation index, conversation handoff, project brief, and build config
- test(FN-019): complete Step 4 — doc validation tests, project-brief.md, and fix mvp-scope placeholders
- feat(FN-019): complete Step 3 — draft mvp-scope.md with milestones, dependency order, and open questions
- docs(FN-019): fix auth callback flow, Python syntax, dev bypass clarity, add AccessProvider protocol and type stubs
- feat(FN-019): complete Step 2 — draft enhanced architecture.md with all 18 required sections

Files changed:
docs/README.md                  |   11 +-
 docs/architecture.md            | 1185 ++++++++++++++++++++++++++++++++++-----
 docs/conversation-handoff.md    |   68 +++
 docs/mvp-scope.md               |  184 ++++++
 docs/project-brief.md           |   31 +
 package.json                    |    3 +
 tests/docs/__init__.py          |    0
 tests/docs/test_architecture.py |  120 ++++
 8 files changed, 1444 insertions(+), 158 deletions(-)

Fusion-Task-Id: FN-019

docs/project-brief.md

@@ -0,0 +1,31 @@
+# Project Brief: Headquarter
+
+## What
+
+Headquarter is a hosted workspace and tool-orchestration platform. Authenticated users create Git-backed projects and launch containerized development tools—starting with RunFusion and code-server—each exposed via its own HTTPS subdomain.
+
+## Why
+
+Cloud IDEs and CI dashboards are convenient but lock users into proprietary platforms. Headquarter gives developers the same convenience with full control over their runtime environments, source code, and routing.
+
+## Who
+
+- Individual developers who want self-hosted workspaces
+- Small teams that outgrow cloud IDEs but do not want to build orchestration from scratch
+- Operators who prefer Docker Compose and Traefik over Kubernetes for simple deployments
+
+## Confirmed Stack
+
+- **Frontend:** React + Vite + TypeScript
+- **Backend:** FastAPI + SQLAlchemy 2.0 + Pydantic v2
+- **Database:** PostgreSQL 17
+- **Auth:** Authentik OIDC
+- **Runtime:** Docker Compose (Portainer-managed)
+- **Routing:** Traefik subdomain-based
+
+## Canonical Documentation
+
+- [Architecture](architecture.md) — System design, domain model, and provider contracts
+- [MVP Scope](mvp-scope.md) — In-scope features, non-goals, milestones, and dependency order
+- [Development](development.md) — Local setup and day-to-day commands
+- [Deployment](deployment.md) — Production deployment assumptions and operator guide