docs(css,ai): update custom property rules and migration skill coverage

[5t3ph]

Description

Three areas of work combined on this branch:

CSS stylelint and linting updates:

• Adds transition-behavior to the stylelint property order so it sorts correctly within the transition-* group
• Adds the full set of CSS system color keywords (ButtonFace, CanvasText, GrayText, etc.) as allowed values in stylelint so forced-colors overrides don't trip the design token enforcement rule

CSS style guide additions (CONTRIBUTOR-DOCS/02_style-guide/01_css/):

• 01_component-css.md: Documents section banner format, expands the :host exception with code examples covering UA resets, entry/exit transitions, and positioning surfaces; adds inherit guidance for sub-elements that track a variant value; clarifies native pseudo-class preference over attribute selectors for states
• 02_custom-properties.md: Adds explicit rules for private property placement (:host does not encapsulate --_swc-*), exposure criteria (expose only when the component itself overrides, with nested component relationship exception), size variant pattern (single property overridden per size selector), state-specific custom property pattern for native browser states on internal elements, and full property name convention
• 03_component-css-pr-checklist.md: Adds a Custom properties section covering all five new rules as checkable items
• 05_anti-patterns.md: Updates anti-pattern # 6 with the nested component relationship exception; adds new anti-pattern # 10 covering size-specific custom properties

AI migration skill and rule updates (.ai/):

• migration-phase-awareness.md: Adds "Migration plan checklist" as an explicit phase-completion step and surfaces it in the Migration Checkpoint format
• migration-styling SKILL.md: Adds Step 5 to write @cssprop JSDoc tags on the SWC class for every exposed property after writing CSS
• migration-documentation SKILL.md: Adds a @cssprop verification step to Phase 7 cross-check
• consumer-migration-guide SKILL.md and prompt: Scopes guide content to what actually shipped in the source, not migration plan categories (Additive/Deferred reflect intent, not final state)
• code-conformance.md: Makes the CSS PR checklist the first and explicit verification step rather than a passive reference

Motivation and context

Custom property exposure decisions were underdocumented, leading to inconsistent implementations across components — in particular: private properties declared on :host (which exposes them), size-specific custom properties that bloat the API surface, and missing @cssprop tags that break Storybook API docs. The PR feedback on Button that prompted these additions also identified that state-specific custom properties (:hover, :focus-visible, :active) on internal elements are correct and necessary because static-color compound overrides require them.

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Latest commit: 78c40f4

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[cdransf]

This looks awesome! ✨

[github-actions]

📚 Branch Preview Links

• Documentation Site (first-gen)
• Storybook (first-gen)
• Storybook (second-gen)

🔍 First Generation Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-6350

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