feat(skills): add gsap-effects skill with typewriter pattern (#158)

* feat(skills): add gsap-effects skill with typewriter pattern

Distills typewriter text animation into a reusable reference:
basic typewriter, blinking cursor, word rotation, appending words,
and a characters-per-second timing guide. Uses GSAP TextPlugin.

Also references the new skill from compose-video.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(skills): emphasize cursor must always blink when idle and sit flush

Two key rules added to the typewriter skill:
1. Cursor must blink in every idle state (after typing, after clearing,
   during hold pauses) — a solid idle cursor looks broken.
2. No whitespace between text and cursor elements in HTML — any gap
   between the last character and the caret looks wrong.

Also adds cursor-hide state for multi-line handoffs and updates word
rotation example to include cursor state management.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(skills): backspace must delete from end, not front

TextPlugin's text:{value:""} removes characters from the front,
which looks wrong. Added a backspace helper that steps through
substrings from right to left using tl.call(). Updated word
rotation example to use it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(skills): handoffs must blink before typing, use margin for spacing

Two lessons from testing:
1. Cursor handoffs need a blink pause — going hide→solid directly
   skips the idle state. Pattern: hide→blink→pause→solid→type→blink.
2. Use margin-left on a wrapper span for spacing between static and
   dynamic text. Flex gap spaces the cursor away, trailing spaces
   collapse.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(skills): enforce single visible cursor as a hard rule

Only one cursor may be visible at a time. Multiple cursors on
screen looks broken. Every other cursor must be cursor-hide.
Promoted to rule #1 in the cursor section.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills/hyperframes-compose/SKILL.md

@@ -139,6 +139,7 @@ Video must be `muted playsinline`. Audio is always a separate `<audio>` element:
 
 For PiP, title cards, and slide show patterns, see [patterns.md](./patterns.md).
 For data, stats, and infographics, see [data-in-motion.md](./data-in-motion.md).
+For typewriter text and other GSAP animation effects, see the `gsap-effects` skill.
 
 ## Output Checklist