docs: add canonical IA landing + first-pass core pages

docs/04-repo-tour.md

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+# Repo tour
+
+High-level map of the codebase so you can quickly find where to change things.
+
+## Top-level
+- `backend/` — FastAPI backend (API server)
+- `frontend/` — Next.js frontend (web UI)
+- `docs/` — documentation
+- `compose.yml` — local/self-host stack (db + backend + frontend)
+- `scripts/` — helper scripts
+
+## Backend: where to look
+- App entrypoint + router wiring: `backend/app/main.py`
+- Routers: `backend/app/api/*`
+- Settings/config: `backend/app/core/config.py`
+- Auth (Clerk + agent token): `backend/app/core/auth.py`, `backend/app/core/agent_auth.py`
+- Models: `backend/app/models/*`
+- Services/domain logic: `backend/app/services/*`
+
+## Frontend: where to look
+- Routes (App Router): `frontend/src/app/*`
+- Components: `frontend/src/components/*`
+- API utilities: `frontend/src/lib/*` and `frontend/src/api/*`
+- Auth (Clerk gating/wrappers): `frontend/src/auth/*`
+
+## Where to change X
+
+| You want to… | Start here |
+|---|---|
+| Add/modify an API endpoint | `backend/app/api/*` + `backend/app/main.py` |
+| Change auth behavior | `backend/app/core/auth.py` + `frontend/src/auth/*` |
+| Change a UI page | `frontend/src/app/*` |
+| Update Compose topology | `compose.yml` |
+
+Next: see [Architecture](05-architecture.md) for system-level flows.