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/visual-design.md

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+# Visual Design Principles
+
+## 12 Core Principles
+
+1. **Geometry Before Algebra** — Show the shape first, the equation second.
+2. **Opacity Layering** — PRIMARY=1.0, CONTEXT=0.4, GRID=0.15. Direct attention through brightness.
+3. **One New Idea Per Scene** — Each scene introduces exactly one concept.
+4. **Spatial Consistency** — Same concept occupies the same screen region throughout.
+5. **Color = Meaning** — Assign colors to concepts, not mobjects. If velocity is blue, it stays blue.
+6. **Progressive Disclosure** — Show simplest version first, add complexity incrementally.
+7. **Transform, Don't Replace** — Use Transform/ReplacementTransform to show connections.
+8. **Breathing Room** — `self.wait(1.5)` minimum after showing something new.
+9. **Visual Weight Balance** — Don't cluster everything on one side.
+10. **Consistent Motion Vocabulary** — Pick a small set of animation types and reuse them.
+11. **Dark Background, Light Content** — #1C1C1C to #2D2B55 backgrounds maximize contrast.
+12. **Intentional Empty Space** — Leave at least 15% of the frame empty.
+
+## Layout Templates
+
+### FULL_CENTER
+One main element centered, title above, note below.
+Best for: single equations, single diagrams, title cards.
+
+### LEFT_RIGHT
+Two elements side by side at x=-3.5 and x=3.5.
+Best for: equation + visual, before/after, comparison.
+
+### TOP_BOTTOM
+Main element at y=1.5, supporting content at y=-1.5.
+Best for: concept + examples, theorem + cases.
+
+### GRID
+Multiple elements via `arrange_in_grid()`.
+Best for: comparison matrices, multi-step processes.
+
+### PROGRESSIVE
+Elements appear one at a time, arranged DOWN with aligned_edge=LEFT.
+Best for: algorithms, proofs, step-by-step processes.
+
+### ANNOTATED_DIAGRAM
+Central diagram with floating labels connected by arrows.
+Best for: architecture diagrams, annotated figures.
+
+## Color Palettes
+
+### Classic 3B1B
+```python
+BG="#1C1C1C"; PRIMARY=BLUE; SECONDARY=GREEN; ACCENT=YELLOW; HIGHLIGHT=RED
+```
+
+### Warm Academic
+```python
+BG="#2D2B55"; PRIMARY="#FF6B6B"; SECONDARY="#FFD93D"; ACCENT="#6BCB77"
+```
+
+### Neon Tech
+```python
+BG="#0A0A0A"; PRIMARY="#00F5FF"; SECONDARY="#FF00FF"; ACCENT="#39FF14"
+```
+
+## Font Selection
+
+Manim's default `Text()` uses the system's default sans-serif font, which often renders with poor kerning. Always specify a font explicitly.
+
+### Recommended Fonts
+
+| Use case | Font | Fallback |
+|----------|------|----------|
+| Body text, titles | `"Inter"`, `"SF Pro Display"` | `"Helvetica Neue"`, `"Arial"` |
+| Code, terminal | `"JetBrains Mono"`, `"SF Mono"` | `"Menlo"`, `"Courier New"` |
+| Math labels | Use `MathTex` (renders via LaTeX, not system fonts) | — |
+
+```python
+# Clean body text
+title = Text("Gradient Descent", font_size=48, font="Inter", weight=BOLD)
+
+# Monospaced code
+code_label = Text("loss.backward()", font_size=24, font="JetBrains Mono")
+
+# Math — always use MathTex, not Text
+equation = MathTex(r"\nabla L = \frac{\partial L}{\partial w}")
+```
+
+### Font Availability
+
+Not all fonts are installed on all systems. Manim falls back silently to a default if the font is missing. Use widely available fonts:
+- **macOS**: SF Pro Display, SF Mono, Menlo, Helvetica Neue
+- **Linux**: DejaVu Sans, Liberation Sans, Ubuntu, Noto Sans
+- **Cross-platform**: Inter (install via Google Fonts), JetBrains Mono (install from jetbrains.com)
+
+For maximum portability, use `"Helvetica Neue"` (body) and `"Menlo"` (code) — both available on macOS and have Linux equivalents.
+
+### Fine-Grained Text Control
+
+`Text()` does not support `letter_spacing` or kerning parameters. For fine control, use `MarkupText` with Pango attributes:
+
+```python
+# Letter spacing (Pango units: 1/1024 of a point)
+MarkupText('<span letter_spacing="6000">HERMES</span>', font_size=18, font="Menlo")
+
+# Bold specific words
+MarkupText('This is <b>important</b>', font_size=24)
+
+# Color specific words
+MarkupText('Red <span foreground="#FF6B6B">warning</span>', font_size=24)
+```
+
+### Text Rendering Quality
+
+Manim's text rendering quality depends heavily on output resolution. At `-ql` (480p), text kerning looks noticeably poor. Always preview text-heavy scenes at `-qm` (720p) or higher. See `references/rendering.md` for quality preset guidance.
+
+## Visual Hierarchy Checklist
+
+For every frame:
+1. What is the ONE thing to look at? (brightest/largest)
+2. What is context? (dimmed to 0.3-0.4)
+3. What is structural? (dimmed to 0.15)
+4. Enough empty space? (>15%)
+5. All text readable at phone size?