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/graphs-and-data.md

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+# Graphs, Plots, and Data Visualization
+
+## Axes
+
+```python
+axes = Axes(
+    x_range=[-3, 3, 1], y_range=[-2, 2, 1],
+    x_length=8, y_length=5,
+    axis_config={"include_numbers": True, "font_size": 24}
+)
+axes.set_opacity(0.15)  # structural element
+x_label = axes.get_x_axis_label(r"x")
+```
+
+## Plotting
+
+```python
+graph = axes.plot(lambda x: x**2, color=BLUE)
+graph_label = axes.get_graph_label(graph, label=r"x^2", x_val=2)
+area = axes.get_area(graph, x_range=[0, 2], color=BLUE, opacity=0.3)
+```
+
+## Animated Plotting
+
+```python
+self.play(Create(graph), run_time=3)  # trace the graph
+
+# Moving dot along curve
+dot = Dot(color=YELLOW).move_to(axes.c2p(0, 0))
+self.play(MoveAlongPath(dot, graph), run_time=3)
+
+# Dynamic parameter
+tracker = ValueTracker(1)
+dynamic = always_redraw(lambda: axes.plot(lambda x: tracker.get_value() * x**2, color=BLUE))
+self.add(dynamic)
+self.play(tracker.animate.set_value(3), run_time=2)
+```
+
+## Bar Charts
+
+```python
+chart = BarChart(
+    values=[4, 6, 2, 8, 5], bar_names=["A", "B", "C", "D", "E"],
+    y_range=[0, 10, 2], bar_colors=[RED, GREEN, BLUE, YELLOW, PURPLE]
+)
+self.play(Create(chart), run_time=2)
+self.play(chart.animate.change_bar_values([6, 3, 7, 4, 9]))
+```
+
+## Number Lines
+
+```python
+nl = NumberLine(x_range=[0, 10, 1], length=10, include_numbers=True)
+pointer = Arrow(nl.n2p(3) + UP * 0.5, nl.n2p(3), color=RED, buff=0)
+tracker = ValueTracker(3)
+pointer.add_updater(lambda m: m.put_start_and_end_on(
+    nl.n2p(tracker.get_value()) + UP * 0.5, nl.n2p(tracker.get_value())))
+self.play(tracker.animate.set_value(8), run_time=2)
+```
+
+## Animated Counters
+
+```python
+counter = DecimalNumber(0, font_size=72, num_decimal_places=0)
+self.play(counter.animate.set_value(1000), run_time=3, rate_func=rush_from)
+```
+
+## Algorithm Visualization Pattern
+
+```python
+values = [5, 2, 8, 1, 9, 3]
+bars = VGroup(*[
+    Rectangle(width=0.6, height=v * 0.4, color=BLUE, fill_opacity=0.7)
+    for v in values
+]).arrange(RIGHT, buff=0.2, aligned_edge=DOWN).move_to(ORIGIN)
+self.play(LaggedStart(*[GrowFromEdge(b, DOWN) for b in bars], lag_ratio=0.1))
+# Highlight, swap, etc.
+```
+
+## Data Story Pattern
+
+```python
+# Before/After comparison
+before = BarChart(values=[3, 5, 2], bar_colors=[RED]*3).shift(LEFT * 3)
+after = BarChart(values=[8, 9, 7], bar_colors=[GREEN]*3).shift(RIGHT * 3)
+self.play(Create(before)); self.wait(1)
+self.play(Create(after)); self.wait(1)
+arrow = Arrow(before.get_right(), after.get_left(), color=YELLOW)
+label = Text("+167%", font_size=36, color=YELLOW).next_to(arrow, UP)
+self.play(GrowArrow(arrow), Write(label))
+```