#24589 Adopt shared design tokens and ratchet lint

Summary

Adopt the design-audit spacing, control, radius, and glass aliases in shared webview CSS, add native CmuxNextDesignTokens, and enforce a ratcheting design-token lint. Existing violations are recorded in a baseline; new off-scale padding/gap/height values and opaque glass fills fail, while baseline reductions require an explicit shrink.

The docs contract is recorded in the paired leo-workspace change design/tokens.md.

Leo’s adopted direction: “I think that's probably fine for the pixels and whatnot. Like, I'll just— I might just go with what you guys have. I can't really be bothered to push pixels on my own.”

Testing

• bun scripts/check-design-tokens.ts — pass
• python3 scripts/cmux-next/prepush-gate.py — pass
• git diff --check — pass
• Focused ratchet test added; Bun test setup is blocked by missing @base-ui/utils/useIsoLayoutEffect in the checkout.

Scope

Composer row geometry is unchanged. Native builds remain fleet-only per repository policy.

agent-identity footer

No capture: this PR only aliases existing native popup values and adds static lint; it does not change rendered native UI.