feat(skill): tailwind-syntax-responsive

Side-by-side v3/v4 coverage of viewport breakpoints and container queries.
Reinforces D-008 dual columns: container queries moved from a v3 plugin
(@tailwindcss/container-queries) to v4 core, with two extra small sizes
(@3xS and @2xs) added.

Files:
- SKILL.md (424 lines) : decision tree viewport vs container, full
  breakpoint and @sm-@7xl scales, named containers, cq units
- references/methods.md : every variant, default scales, custom config
  patterns for v3 + v4, browser baseline
- references/examples.md : setup + 16 patterns side-by-side incl. real
  responsive card both ways, migration v3-to-v4
- references/anti-patterns.md : 15 traps incl. mobile-first reversal,
  v3 plugin missing or v4 plugin lingering, named-container bleed,
  cqb without @container-size

Verified against:
- https://tailwindcss.com/docs/responsive-design (v4)
- https://github.com/tailwindlabs/tailwindcss-container-queries (v3 plugin + deprecation)
- https://v3.tailwindcss.com/docs/responsive-design

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: FreekHeijting