Simplify CSS variable system, add css-data.json for IDE/LLM support

[amcclain]

Summary

• Removes the two-tier CSS custom property override system. Previously, Hoist defined framework variables like --xh-grid-bg: var(--grid-bg, var(--xh-bg)), where the unprefixed --grid-bg served as an app-level override hook. This added complexity, confused IDE autocomplete (apps were expected to set vars that didn't appear in any source), and was inconsistently applied across the codebase. Applications now override the --xh- prefixed variables directly - simpler, IDE-friendly, and consistent with modern CSS variable conventions.

• Adds css-data.json for IDE autocomplete and LLM/agent consumption. A VS Code Custom Data file generated from vars.scss by bin/generate-css-data.mjs, providing autocomplete and hover documentation for all ~440 --xh-* CSS custom properties. Descriptions are auto-generated from variable name structure and value relationships (derivations, aliases, intent system, palette colors), with support for explicit /// doc comments that take precedence. A pre-commit hook validates the file stays in sync when SCSS files change; prepack regenerates before publish.

Motivation

The unprefixed override hooks were an unnecessary layer of indirection. Analysis of sample apps found only ~39% of the 265 hooks were ever used, and apps that did use them got no IDE support - the unprefixed names existed only inside var() fallback expressions in vars.scss, invisible to autocomplete. Meanwhile, the --xh- prefixed names were the ones actually documented and referenced in component SCSS. Removing the two-tier pattern and standardizing on --xh- eliminates confusion and makes the system work better with modern tooling.

The css-data.json file takes this further by giving VS Code users immediate autocomplete with descriptions, and - more importantly - giving coding agents and LLMs a machine-readable index of every available CSS variable with relationship and override guidance baked into the descriptions.

Breaking change

This is a low-difficulty mechanical migration for apps. Most overrides simply need an xh- prefix added. A small number of unprefixed names differed from their --xh- counterparts - the upgrade notes include a complete mapping table and grep commands for finding affected code.

See docs/upgrade-notes/v87-upgrade-notes.md for step-by-step instructions.

Originally drafted against v83; merged up to v87-SNAPSHOT. The intent text-color copy-paste bug fix that originally accompanied this work was merged separately and shipped in v83.0.0, so it is no longer part of this PR.

Test plan

• Application loads without console errors in both light and dark themes
• Custom colors, spacing, and typography render correctly after migrating overrides
• css-data.json provides autocomplete in VS Code when configured via css.customData
• Pre-commit hook blocks commits with stale css-data.json when SCSS is modified
• node bin/generate-css-data.mjs --check passes after regeneration

[amcclain]

We have a further option to also remove the indirection we have with unit-less values, then we then multiply to make varying combinations (e.g. --xh-pad vs --xh-pad-px). This could also be simplified out.

We should discuss if we want to make that change also, or if this is already pushed as far as we're comfortable.

I do like that this cleans up something that's been muddy in my mind from the very beginning and would love to get it merged when we're ready.

Feedback + thoughts appreciated!