skills: adopt GSAP Layer 3 + Layer 2 animation-runtime routing

Mirror the official GSAP AI skills (greensock/gsap-skills, MIT) into
.agents/skills/ and add the Layer 2 wiring that makes them discoverable
from a fresh context.

GSAP covers animation needs that Remotion primitives strain at:
per-character text reveals (SplitText), SVG shape morphs (MorphSVG),
curved camera paths (MotionPath), stroke-reveal line drawing (DrawSVG),
layout-to-layout flight (Flip), and custom bezier easings (CustomEase).
Also becomes mandatory day-1 knowledge if we wire in HyperFrames later
(HF uses GSAP timelines as its native animation runtime).

Layer 3 adds (.agents/skills/):
- gsap-core, gsap-timeline, gsap-plugins, gsap-utils
- gsap-react, gsap-performance
- gsap-scrolltrigger, gsap-frameworks (situational)
- gsap/README.md — OpenMontage-specific framing and Remotion-safe usage

Layer 2 wiring (the discovery triggers):
- skills/meta/animation-runtime-selector.md — NEW routing meta-skill.
  Decision matrix covering Remotion primitives, GSAP plugins, framer-
  motion, Lottie, Manim, D3, TerminalScene. Enforces the "keep it
  simple" bias: reach for GSAP only when the plugin genuinely earns
  its bundle weight.
- skills/pipelines/explainer/asset-director.md — references GSAP for
  kinetic typography, multi-step choreography, SVG line draws.
- skills/pipelines/animation/asset-director.md — references GSAP for
  logo morphs, motion paths, FLIP transitions, custom easings.
- skills/pipelines/cinematic/asset-director.md — references GSAP for
  cinematic camera moves, per-char title reveals, prestige easings.
- AGENT_GUIDE.md — adds a categorized Layer 3 skills table so a fresh-
  context agent can find the right skill by what they're trying to do,
  plus a pointer to animation-runtime-selector.md for routing.

Determinism: every GSAP use inside Remotion must drive timeline progress
from useCurrentFrame(), never requestAnimationFrame. Three Remotion-safe
patterns are documented in both the gsap/README and the selector skill.

Attribution: https://github.com/greensock/gsap-skills (MIT).

.agents/skills/gsap-timeline/SKILL.md

@@ -0,0 +1,107 @@
+---
+name: gsap-timeline
+description: Official GSAP skill for timelines — gsap.timeline(), position parameter, nesting, playback. Use when sequencing animations, choreographing keyframes, or when the user asks about animation sequencing, timelines, or animation order (in GSAP or when recommending a library that supports timelines).
+license: MIT
+---
+
+# GSAP Timeline
+
+## When to Use This Skill
+
+Apply when building multi-step animations, coordinating several tweens in sequence or parallel, or when the user asks about timelines, sequencing, or keyframe-style animation in GSAP.
+
+**Related skills:** For single tweens and eases use **gsap-core**; for scroll-driven timelines use **gsap-scrolltrigger**; for React use **gsap-react**.
+
+## Creating a Timeline
+
+```javascript
+const tl = gsap.timeline();
+tl.to(".a", { x: 100, duration: 1 })
+  .to(".b", { y: 50, duration: 0.5 })
+  .to(".c", { opacity: 0, duration: 0.3 });
+```
+
+By default, tweens are **appended** one after another. Use the **position parameter** to place tweens at specific times or relative to other tweens.
+
+## Position Parameter
+
+Third argument (or position property in vars) controls placement:
+
+- **Absolute**: `1` — start at 1 second.
+- **Relative (default)**: `"+=0.5"` — 0.5s after end; `"-=0.2"` — 0.2s before end.
+- **Label**: `"labelName"` — at that label; `"labelName+=0.3"` — 0.3s after label.
+- **Placement**: `"<"` — start when recently-added animation starts; `">"` — start when recently-added animation ends (default); `"<0.2"` — 0.2s after recently-added animation start.
+
+Examples:
+
+```javascript
+tl.to(".a", { x: 100 }, 0);           // at 0
+tl.to(".b", { y: 50 }, "+=0.5");      // 0.5s after last end
+tl.to(".c", { opacity: 0 }, "<");     // same start as previous
+tl.to(".d", { scale: 2 }, "<0.2");    // 0.2s after previous start
+```
+
+## Timeline Defaults
+
+Pass defaults into the timeline so all child tweens inherit:
+
+```javascript
+const tl = gsap.timeline({ defaults: { duration: 0.5, ease: "power2.out" } });
+tl.to(".a", { x: 100 }).to(".b", { y: 50 }); // both use 0.5s and power2.out
+```
+
+## Timeline Options (constructor)
+
+- **paused: true** — create paused; call `.play()` to start.
+- **repeat**, **yoyo** — same as tweens; apply to whole timeline.
+- **onComplete**, **onStart**, **onUpdate** — timeline-level callbacks.
+- **defaults** — vars merged into every child tween.
+
+## Labels
+
+Add and use labels for readable, maintainable sequencing:
+
+```javascript
+tl.addLabel("intro", 0);
+tl.to(".a", { x: 100 }, "intro");
+tl.addLabel("outro", "+=0.5");
+tl.to(".b", { opacity: 0 }, "outro");
+tl.play("outro");  // start from "outro"
+tl.tweenFromTo("intro", "outro"); // pauses the timeline and returns a new Tween that animates the timeline's playhead from intro to outro with no ease.
+```
+
+## Nesting Timelines
+
+Timelines can contain other timelines.
+
+```javascript
+const master = gsap.timeline();
+const child = gsap.timeline();
+child.to(".a", { x: 100 }).to(".b", { y: 50 });
+master.add(child, 0);
+master.to(".c", { opacity: 0 }, "+=0.2");
+```
+
+## Controlling Playback
+
+- **tl.play()** / **tl.pause()**
+- **tl.reverse()** / **tl.progress(1)** then **tl.reverse()**
+- **tl.restart()** — from start.
+- **tl.time(2)** — seek to 2 seconds.
+- **tl.progress(0.5)** — seek to 50%.
+- **tl.kill()** — kill timeline and (by default) its children.
+
+## Official GSAP Best practices
+
+- ✅ Prefer timelines for sequencing
+- ✅ Use the **position parameter** (third argument) to place tweens at specific times or relative to labels.
+- ✅ Add **labels** with `addLabel()` for readable, maintainable sequencing.
+- ✅ Pass **defaults** into the timeline constructor so child tweens inherit duration, ease, etc.
+- ✅ Put ScrollTrigger on the timeline (or top-level tween), not on tweens inside a timeline.
+
+## Do Not
+
+- ❌ Chain animations with **delay** when a **timeline** can sequence them; prefer `gsap.timeline()` and the position parameter for multi-step animation.
+- ❌ Forget to pass **defaults** (e.g. `defaults: { duration: 0.5, ease: "power2.out" }`) when many child tweens share the same duration or ease.
+- ❌ Forget that **duration** on the timeline constructor is not the same as tween duration; timeline “duration” is determined by its children.
+- ❌ Nest animations that contain a ScrollTrigger; ScrollTriggers should only be on top-level Tweens/Timelines.