feat: add antiPatterns to styling_preflight output

[himerus]

Summary

• Exports buildAntiPatterns from quick-ref.ts and imports it in styling-preflight.ts
• Adds antiPatterns: string[] to PreflightResult interface
• Agents now get positive examples (correctSnippet), validation (issues), AND negative examples (antiPatterns) in a single styling_preflight call
• 4 new tests (26 total for preflight), all passing

Why

The styling_preflight tool is the primary validation tool agents call after writing CSS. It returned validation issues and correct snippets but no explicit "don't do this" examples. Agents kept repeating the same mistakes (descendant selectors into shadow DOM, tokens on :root, ::part() chaining) because they only saw what was wrong with their specific CSS, not the broader patterns to avoid.

Test plan

• 4 new tests verifying antiPatterns presence and content
• Full suite: 2381/2382 pass (pre-existing phantom events failure only)

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: 72c420ee-93a8-4978-8a72-37645c5c8786

📥 Commits

Reviewing files that changed from the base of the PR and between 907399d and 95f6967.

📒 Files selected for processing (6)

• .changeset/preflight-anti-patterns.md
• custom-elements.json
• packages/core/src/handlers/quick-ref.ts
• packages/core/src/handlers/styling-preflight.ts
• tests/__fixtures__/benchmark-results/latest-benchmark.json
• tests/handlers/styling-preflight.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/preflight-anti-patterns

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