⚡ Bolt: [performance improvement] Precompute lowercase strings in todo-tracking

[raccioly]

💡 What:
Optimized the checkUntrackedTodos validator inside cli/validators/todo-tracking.mjs. The content of tracking documents is now converted to lowercase once when the file is initially loaded, rather than being redundantly lowercased inside a .some() loop for every single TODO comment found in the project.

🎯 Why:
Previously, if the project contained N TODO items and M tracking documents, the entire contents of the tracking documents were lowercased N * M times. For large documentation files (like ARCHITECTURE.md or ROADMAP.md) and a codebase with hundreds of TODOs, this created a significant and unnecessary CPU bottleneck during string allocation.

📊 Impact:
Substantially reduces string allocations and garbage collection overhead during the todo-tracking validation phase. The time complexity for the text preparation phase drops from $O(N \times M)$ to $O(M)$.

🔬 Measurement:
Run node --test tests/*.test.mjs to verify functionality remains unchanged. You can benchmark the overall CLI speed improvements on large projects by running time docguard guard before and after this change.

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PR created automatically by Jules for task 2707253010487627082 started by @raccioly

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