[docs-infra] Force raw color values in CSS Modules demos

[aarongarciah]

This prevents incorrect usage of colors in CSS Modules demos. Helpful for LLMs and humans alike.

• Add a new custom Stylelint plugin to catch non-raw colors in CSS Modules demos.
• Update AGENTS.md mentioning the raw-colors requirement.
• Update three demos that were using wrong colors.

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	0B(0.00%)	0B(0.00%)

Details of bundle changes

Performance

Total duration: 1,411.93 ms 🔺+243.76 ms^(+20.9%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 2,132.89 ms +347.70 ms^(+19.5%)

Test	Duration	Renders
Tabs mount (200 instances)	271.57 ms 🔺+51.15 ms(+23.2%)	4 (+0)
Slider mount (300 instances)	197.03 ms 🔺+39.79 ms(+25.3%)	3 (+0)
Menu mount (300 instances)	170.97 ms 🔺+32.77 ms(+23.7%)	2 (+0)
Select mount (200 instances)	170.38 ms 🔺+30.09 ms(+21.4%)	3 (+0)
Scroll Area mount (300 instances)	109.07 ms 🔺+18.55 ms(+20.5%)	3 (+0)

…and 3 more (+4 within noise) — details

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Check out the code infra dashboard for more information about this PR.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4865
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4865
  

commit: a21f2a7

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