[codex] Widen article guides and tighten Learn spacing

[mberman84]

What changed

• widened the shared article layout used by Learn and For agents
• increased readable content widths for headings, body copy, agent steps, cards, and resource grids
• reduced the compounded gap between the Learn introduction and its first section
• cache-busted the stylesheet on both affected pages and added regression assertions

Why

The Learn and For agents pages were constrained to a narrow desktop column, and Learn stacked header padding with section margin to create an oversized empty gap after the introduction.

Validation

• node scripts/check.mjs
• JavaScript syntax checks for site and build scripts
• npm --prefix worker run check (14 tests)
• JSON validation for site data and SEO benchmark
• git diff --check