docs: restructure TASKS.md into a catalog — one file per task in TASKS/

TASKS.md is now the index (status table, open items, research pointer).
33 files: 32 task files + 1 research facts file. The full take-saga
narrative and all design decisions are preserved verbatim; the catalog
makes the queue readable without opening every task body. Schema and
AGENTS.md updated to reflect the new layout.

TASKS/task-01-initial-scaffold.md

@@ -0,0 +1,15 @@
+# TASK 1 — Initial Scaffold
+
+> Catalog: [`TASKS.md`](../TASKS.md) — status and requirements live here.
+
+
+**Goal:** Working C# / WPF project with MVVM architecture, dark-theme main window, and YouTube service stubs.
+
+### Status: ✅ Done
+
+1. ✅ Models: Scene, Source, StreamConfig, StreamHealth, YouTubeChannel, ChatMessage
+2. ✅ Services: YouTubeAuthService (OAuth2), YouTubeStreamService (broadcast/health), YouTubeChatService (chat polling)
+3. ✅ MainViewModel: scene management, stream controls, chat
+4. ✅ MainWindow: scene/source panel, preview area, chat panel, status bar
+5. ✅ Clean build, 0 warnings (WSL + Windows)
+