feat(leetcode-sync): integrate LeetCode metadata sync and post-processing pipeline

- Add LeetCode problem metadata synchronization with local caching
  - Introduce sync_leetcode_data.py with cache validation and refresh options
  - Store normalized problem data in tools/.cache/leetcode_problems.json
- Establish question_id as the single source of truth across the pipeline
  - Align file naming, API lookup, and post-processing on question_id
- Normalize LeetCode references during post-processing
  - Convert plain text problem mentions into canonical LeetCode URLs
  - Preserve original text when metadata is unavailable
- Automatically attach GitHub solution links when corresponding solutions exist
- Decouple external API access from core logic via a stable data layer

This change turns LeetCode API usage into infrastructure rather than ad-hoc lookups,
enabling reproducible runs, future CI validation, and safe extensibility.

Author: lufftw

.gitignore

@@ -38,6 +38,7 @@ Thumbs.db
 
 # Debug
 *.log
+tools/outputs/debug/run_*_regen_*
 
 # Generated failed test cases (from --save-failed)
 *_failed_*.in

docs/pages/mindmaps/neetcode_ontology_agent_evolved_en.html

@@ -0,0 +1,5 @@
+{
+  "cached_at": "2025-12-15T17:43:40.099954",
+  "total_problems": 3778,
+  "cache_version": "1.0"
+}