refactor(skill): trim techniques.md to actual techniques (13 from 20)

Removed 7 entries that are not techniques. They're either UI style
treatments, hand-rolled CSS templates, or named effects — different
kinds of artifacts that don't belong under the same "primitive
animation techniques" frame:

- 12. Frosted Glass Panels  → style treatment
- 15. Impact Line on Text Drop  → named effect
- 16. Device Mockups (Laptop + Phone)  → hand-rolled CSS template
- 17. Aurora Gradient Backgrounds  → style treatment
- 18. Floating Particles  → style treatment
- 19. Terminal UI with Typing  → hand-rolled CSS template
- 20. Moodboard / Editorial Layout  → hand-rolled layout

The 13 entries that remain (SVG drawing, Canvas 2D, CSS 3D, Per-Word
Kinetic Type, Lottie, Video Compositing, Char Typing, Variable Font,
MotionPath, Velocity-Matched Transitions, Audio-Reactive, Clip-Path
Reveal, WebGL Shader Art) are real primitives — animation building
blocks an agent composes into beats, not finished recipes to lift
verbatim.

Updated the header to say 13 and point at `registry/blocks/` for
pre-built UI templates instead of pretending they're techniques. ToC
table and section numbering both updated. Net: -361 / +6 lines.

Author: ukimsanov