feat: Inject valid file paths into system prompt

[ajianaz]

Summary

Reduce hallucination by injecting actual diff file paths into the system prompt, with post-parse filtering of invalid file references. Referenced from old cora repo anti-hallucination patterns.

Accept Criteria

• Extract file paths from diff before sending to LLM
• Inject 'Valid files in this diff: [list]' into system prompt
• Post-parse: filter out ReviewIssues where file path not in actual diff files
• Log count of filtered invalid issues (debug level)
• Unit test: prompt contains file paths
• Unit test: issue with invalid file path gets filtered

Scope

llm.rs prompt building + parse post-processing