Fix Chromatic configuration drift and Storybook caching

[effinrich]

Summary

• consolidate local and CI Chromatic execution on the root configuration
• remove unsupported GitHub Action inputs and stale package-local configs
• enable Turbo caching for deterministic Storybook builds

Verification

• bun run build-storybook --filter=@richtillman/ui
• consecutive scoped build reuses the Turbo cache
• workflow YAML and Chromatic/Turbo JSON parse successfully

This is intentionally stacked on chromatic-fix because it follows up on that branch's Chromatic pipeline changes.

[chromatic-com]

Tip

All tests passed and all changes approved!

🟢 UI Tests: 38 visual and accessibility changes accepted as baselines
🟢 UI Review: Approved by Rich Tillman
Storybook Publish: 79 stories published

[Copilot]

Copilot review overview

🟢 Approval recommended

Only a minor non-blocking style nit remains.

Review effort: Lite
Findings: None

What changed in this PR

Consolidates Chromatic configuration at the repository root and enables Turbo caching for Storybook builds.

Changes:

• Centralizes Chromatic settings and CI execution.
• Removes stale package-local configuration.
• Enables caching for Storybook builds.

File	Description
turbo.json	Enables Storybook build caching.
packages/​ui/​turbo.json	Removes obsolete package-level configuration.
packages/​ui/​README.md	Documents shared Chromatic configuration; contains a minor semicolon style nit.
packages/​ui/​chromatic.config.json	Removes stale package-local configuration.
chromatic.config.json	Uses the package’s chromatic-build script.
.github/​workflows/​chromatic.yml	Uses the shared root Chromatic configuration.

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[effinrich]

Reviewed in Chromatic: build 45 passes with all 38 changes human-approved and zero accessibility violations. These replace build 43's capture-timeout failures, so no additional code fix is needed.