Add AI/LLM-focused LeetCode study list

[Copilot]

Curates AI-related LeetCode problems aligned to generative AI/LLM fundamentals (tokenization, attention windows, retrieval, sparse math).

• New guide: docs/topics/ai-llm.md lists targeted problems by theme (tokenization, sequence alignment, attention windows, retrieval/similarity search, sparse linear algebra, graph reasoning).
• Docs link: Added AI/LLM topic guide to the README Topic Guides.

Example snippet (from the new guide):

## Tokenization & language primitives
- [0208 - Implement Trie (Prefix Tree)](...) — prefix vocabulary construction for token lookup/autocomplete.
- [0139 - Word Break](...) — DP segmentation akin to validating token boundaries (BPE-like).

Original prompt

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This section details on the original issue you should resolve

<issue_title>LeetCode AI Related Problems</issue_title>
<issue_description>Gather a list of problems in LeetCode that can be considered AI related. Update this issue with the list that has been collected.</issue_description>

<agent_instructions>Based on algorithms that are important to generative AI and LLM's, find problems on LeetCode that would be good to solve for learning AI.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes LeetCode AI Related Problems #66

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