Introduces `find_symbol_references` in `ai_review/git.py` to detect deleted symbols within a diff and search for remaining references across the repository using `git grep`. This provides the LLM with factual context to determine if callers have been properly updated, reducing "hallucinated" risks regarding unsynchronized changes.

Key changes:
- Implements heuristic-based symbol extraction for deleted lines (supporting Kotlin, Java, Swift, JS/TS, etc.).
- Filters out references within the files already modified in the current diff.
- Appends a formatted "Symbol Reference Context" block to both standard and deep review prompts.
- Limits analysis to the first 8 symbols and 20 references per symbol to manage prompt size.