🧹 Improve cluster risk heuristic with content analysis

[saurabhsharma2u]

🎯 What:
The calculateClusterRisk function in plugins/core/src/graph/cluster.ts was updated to use a content-based heuristic instead of a simple size-based one.

💡 Why:
The previous heuristic relied solely on cluster size, which could lead to inaccurate risk assessments (e.g., small clusters of identical pages were marked low risk). By analyzing actual content duplication (Titles and H1s), the risk assessment becomes much more accurate for detecting cannibalization issues.

✅ Verification:

• Created a new test file plugins/core/tests/clustering_risk.test.ts with 5 test cases covering:
  • High risk scenarios (identical titles, identical H1s).
  • Low risk scenarios (unique content in small clusters).
  • Medium risk scenarios (large clusters even with unique content).
  • Fallback behavior when HTML is missing.
• Ran npx vitest run plugins/core/tests/clustering_risk.test.ts and confirmed all tests passed.
• Ran all tests in plugins/core (npx vitest run plugins/core/tests) and confirmed no regressions (170 tests passed).
• Verified TypeScript compilation with npm run build.

✨ Result:
The cluster risk heuristic is now robust and content-aware, providing better insights into SEO issues like duplicate content and keyword cannibalization.

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PR created automatically by Jules for task 6798153227314384451 started by @saurabhsharma2u

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