add PDD workflow: CLAUDE.md, PDD.md, sprint spec with active symlink

CLAUDE.md

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+# T YouTube — Text-Based YouTube Digest
+
+## PDD Workflow
+
+This project follows **PDD (Product Definition Document)** development.
+
+**Always read these files before writing code:**
+- `docs/PDD.md` — product vision, goals, targets (the north star)
+- `docs/specs/active.md` — current sprint spec (what we're building now)
+
+**Workflow:**
+1. PDD → Sprint Spec → Implementation → Validate ACs
+2. Scope changes update the sprint spec first, code second
+3. Never ship without ACs passing
+
+## Dev Commands
+
+- **Build:** `docker compose up -d --build`
+- **Test:** `./run_tests.sh`
+- **Run locally:** `uvicorn backend.main:app --reload` (backend) + `npm run dev` (frontend)
+- **Sync subscriptions:** `python run_sync.py`
+
+## Architecture
+
+- **Frontend:** Svelte 5 + Vite (`frontend/`)
+- **Backend:** FastAPI + SQLite (`backend/`)
+- **Deployment:** Docker container on NAS DS224+, port 8001
+- **Sync:** `run_sync.py` fetches YouTube subscriptions + transcripts via yt-dlp
+- **Auth:** Authelia SSO in front, cookie-based session
+- **Proxy:** YouTube blocked from China — sync runs via VPS (Oracle Tokyo) or proxy
+
+## Key Files
+
+- `docs/architecture.md` — full architecture reference
+- `Dockerfile` — multi-stage build (node → python)
+- `docker-compose.yml` — deployment config
+- `run_sync.py` — subscription sync entry point
+- `config/cookies.txt` — YouTube auth cookies (not committed)