[3.0] Document the theme's CSS naming conventions

[albertlast]

Description

Documents the theme's CSS naming conventions in AGENTS.md, so that coding agents get them right the first time instead of being corrected in review.

php-cs-fixer only looks at PHP, so none of this is enforced automatically and nothing in the repository stated it. This came up in #9372, where the review asked for snake_case class names instead of kebab-case.

The new ### CSS naming subsection records three things, each checked against release-3.0 rather than assumed:

• Class names are snake_case. 389 of the 392 class selectors in Themes/default/css/index.css follow this. The three that do not (.sceditor-container, .dz-image-preview, .g-recaptcha) are all owned by third-party code, so the section says explicitly that they are not a precedent for new classes — otherwise the next reader finds a kebab-case class and copies it.
• Custom properties stay kebab-case (--body-bg, --primary-color-500), with a note not to "correct" them. The class-name rule is about class names only, and there are 209 kebab-case custom properties already in the theme.
• The underscore-suffix pattern for custom properties, --component-property_variant. The variant is usually a state (--input-bg_hover, --button-border-color_active), but the same slot carries other modifiers where a component needs them (--progressbar-inner-bg_green, --genericbar-inner-box-shadow_vertical). 37 of the 246 custom properties use a suffix.

Documentation only — no code, CSS or template changes, so nothing here affects the running forum.

Issues References (Fixes|Related|Closes)

1. Related: [3.0] Theme split (wave 3, part 4) — Give the width-limited wrappers a shared class #9372