feat(skills): add manim-video skill for mathematical and technical animations

Production pipeline for creating 3Blue1Brown-style animated videos
using Manim Community Edition. The agent handles the full workflow:
creative planning, Python code generation, rendering, scene stitching,
audio muxing, and iterative refinement.

Modes: concept explainers, equation derivations, algorithm
visualizations, data stories, architecture diagrams, paper explainers,
3D visualizations.

9 reference files, setup verification script, README.
All API references verified against ManimCommunity/manim source.

skills/creative/manim-video/references/rendering.md

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+# Rendering Reference
+
+## Prerequisites
+
+```bash
+manim --version       # Manim CE
+pdflatex --version    # LaTeX
+ffmpeg -version       # ffmpeg
+```
+
+## CLI Reference
+
+```bash
+manim -ql script.py Scene1 Scene2    # draft (480p 15fps)
+manim -qm script.py Scene1           # medium (720p 30fps)
+manim -qh script.py Scene1           # production (1080p 60fps)
+manim -ql --format=png -s script.py Scene1  # preview still (last frame)
+manim -ql --format=gif script.py Scene1     # GIF output
+```
+
+## Quality Presets
+
+| Flag | Resolution | FPS | Use case |
+|------|-----------|-----|----------|
+| `-ql` | 854x480 | 15 | Draft iteration (layout, timing) |
+| `-qm` | 1280x720 | 30 | Preview (use for text-heavy scenes) |
+| `-qh` | 1920x1080 | 60 | Production |
+
+**Text rendering quality:** `-ql` (480p15) produces noticeably poor text kerning and readability. For scenes with significant text, preview stills at `-qm` to catch issues invisible at 480p. Use `-ql` only for testing layout and animation timing.
+
+## Output Structure
+
+```
+media/videos/script/480p15/Scene1_Intro.mp4
+media/images/script/Scene1_Intro.png  (from -s flag)
+```
+
+## Stitching with ffmpeg
+
+```bash
+cat > concat.txt << 'EOF'
+file 'media/videos/script/480p15/Scene1_Intro.mp4'
+file 'media/videos/script/480p15/Scene2_Core.mp4'
+EOF
+ffmpeg -y -f concat -safe 0 -i concat.txt -c copy final.mp4
+```
+
+## Add Voiceover
+
+```bash
+# Mux narration
+ffmpeg -y -i final.mp4 -i narration.mp3 -c:v copy -c:a aac -b:a 192k -shortest final_narrated.mp4
+
+# Concat per-scene audio first
+cat > audio_concat.txt << 'EOF'
+file 'audio/scene1.mp3'
+file 'audio/scene2.mp3'
+EOF
+ffmpeg -y -f concat -safe 0 -i audio_concat.txt -c copy full_narration.mp3
+```
+
+## Add Background Music
+
+```bash
+ffmpeg -y -i final.mp4 -i music.mp3 \
+  -filter_complex "[1:a]volume=0.15[bg];[0:a][bg]amix=inputs=2:duration=shortest" \
+  -c:v copy final_with_music.mp4
+```
+
+## GIF Export
+
+```bash
+ffmpeg -y -i scene.mp4 \
+  -vf "fps=15,scale=640:-1:flags=lanczos,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" \
+  output.gif
+```
+
+## Aspect Ratios
+
+```bash
+manim -ql --resolution 1080,1920 script.py Scene  # 9:16 vertical
+manim -ql --resolution 1080,1080 script.py Scene  # 1:1 square
+```
+
+## Render Workflow
+
+1. Draft render all scenes at `-ql`
+2. Preview stills at key moments (`-s`)
+3. Fix and re-render only broken scenes
+4. Stitch with ffmpeg
+5. Review stitched output
+6. Production render at `-qh`
+7. Re-stitch + add audio