Principle-based system prompt, recall boost, post-processing dedup

System prompt: Replace 13 hyper-specific bug categories with 5 investigation
principles (trace both branches, verify call-site contracts, concurrency
audit, test rigor, root-cause dedup). Keep categories as concise reference.

Review prompt: Remove "quality over quantity" that suppressed recall. Add
"report every bug, even borderline" and "check every changed file" rules.

Pipeline: Add post-processing dedup that merges issues with same category
and >35% keyword overlap. Increase max_tool_rounds from 25 to 40.

10-PR eval: F1 63.2% (P=92.3%, R=48.0%) vs baseline 51.0%.