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-15-scene-backgrounds.md

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+# TASK 15 — Stock scene background images
+
+> Catalog: [`TASKS.md`](../TASKS.md) — status and requirements live here.
+
+
+**Goal:** branded background images for each canonical scene (Starting, Live, BRB, Chat, Ending)
+that both provide working hints to creators and self-promote LlamaCasty.
+
+### Status: ✅ Shipped — all 5 scenes seeded
+
+1. ✅ **Starting backdrop** — `Assets/starting-backdrop.jpg` (embedded resource), `SeedStartingBackdrop()` in MainViewModel, idempotent
+2. ✅ **BRB backdrop** — `Assets/brb-backdrop.jpg` (user-provided Gemini image), `SeedBrbBackdrop()` in MainViewModel, idempotent
+3. ✅ **Live backdrop** — `Assets/live-backdrop.jpg` (user-provided image), seeded via `SeedLiveBackdropAsset()`, used as the static fallback (TASK 16)
+4. ✅ **Ending backdrop** — `Assets/ending-backdrop.jpg` (user-provided image), `SeedEndingBackdrop()` replaces existing (not skip-if-exists)
+5. ✅ **Chat backdrop** — `Assets/chat-backdrop.jpg` (user-provided image), `SeedChatBackdrop()` at startup
+6. ✅ `EnsureDefaultBackdrop()` called on every scene switch — seeds the default backdrop if scene has none
+
+### Design decisions
+
+- Images are user-provided, not generated — the creator owns the brand look
+- Each scene gets its own image (Starting = "Starting Soon", Live = live branding, BRB = "Be Right Back", etc.)
+- The images double as the no-game backdrop fallback (TASK 16), so the Live scene image should work as a desktop replacement
+