Reorganize docs and add implementation spec

Move GOAL.md, PRODUCT.md, SPEC.md from repo root into doc/. Add
AGENTS.md (contributor guidance), doc/DEVELOPING.md (dev setup),
doc/SPEC-implementation.md (V1 implementation contract), and
doc/specs/ui.md (UI design spec). Update ClipHub doc.

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

doc/DEVELOPING.md

@@ -0,0 +1,58 @@
+# Developing
+
+This project can run fully in local dev without setting up PostgreSQL manually.
+
+## Prerequisites
+
+- Node.js 20+
+- pnpm 9+
+
+## Start Dev
+
+From repo root:
+
+```sh
+pnpm install
+pnpm dev
+```
+
+This starts:
+
+- API server: `http://localhost:3100`
+- UI: `http://localhost:5173`
+
+## Database in Dev (Auto-Handled)
+
+For local development, leave `DATABASE_URL` unset.
+The server will automatically use embedded PGlite and persist data at:
+
+- `./data/pglite`
+
+No Docker or external database is required for this mode.
+
+## Quick Health Checks
+
+In another terminal:
+
+```sh
+curl http://localhost:3100/api/health
+curl http://localhost:3100/api/companies
+```
+
+Expected:
+
+- `/api/health` returns `{"status":"ok"}`
+- `/api/companies` returns a JSON array
+
+## Reset Local Dev Database
+
+To wipe local dev data and start fresh:
+
+```sh
+rm -rf data/pglite
+pnpm dev
+```
+
+## Optional: Use External Postgres
+
+If you set `DATABASE_URL`, the server will use that instead of embedded PGlite.