Fix gap #22 and add CinematicRenderer captions + music support

- AGENT_GUIDE.md: add "never read source code" rule — skills are the
  interface, not .py files
- animation.yaml: add Layer 2 skill-first guardrail in assets stage
- video-reference-analyst.md: Step 4b now mandates Layer 2 before Layer 3,
  explicitly forbids reading implementation code
- CinematicRenderer: add TikTok-style CaptionOverlay and separate music
  track support (narration + music as independent audio layers)
- cinematic/types.ts: add CinematicCaptionConfig and music prop types

AGENT_GUIDE.md

@@ -580,8 +580,10 @@ OpenMontage has three instruction layers:
 Reading order:
 
 1. registry / tool contract — discover what's available
-2. relevant pipeline or creative skill — know HOW to use it in this context
-3. underlying vendor skill — **mandatory before calling any generation tool**
+2. relevant pipeline or creative skill (Layer 2) — know HOW to use it in this context
+3. underlying vendor skill (Layer 3) — **mandatory before calling any generation tool**
+
+**NEVER read tool source code (`tools/*.py`) to understand how to use a tool.** Skills exist precisely so you don't need implementation details. Layer 2 tells you *what* and *when*. Layer 3 tells you *how*. If you're reading `.py` files to figure out input schemas or provider options, you're doing it wrong — that information belongs in the skill layer.
 
 **Layer 3 is not optional.** Every generation tool (video, image, TTS, music) has an `agent_skills` field listing its Layer 3 skills. These skills contain provider-specific prompt engineering, parameter tuning, and quality techniques. Read them before writing prompts. The difference between a generic prompt and a skill-informed prompt is the difference between "usable" and "cinematic."