Fix inconsistent test file placement by always auto-detecting location

[hiroshinishio]

Summary

• Always auto-detect test location convention from the actual repo instead of trusting the dashboard testFileLocation default ("Co-located with source"), which was wrong for repos whose tests are actually in separate directories
• Raised dominant-pattern threshold from 60% to 80% so only strong conventions override the co-location default
• Rewrote coding_standards.xml file organization rules: co-location is ideal, follow 80%+ separate pattern, move/merge existing tests toward co-location, test_dir_prefixes is for finding only

Root cause

Many repos had testFileLocation: "Co-located with source" (dashboard default, never intentionally configured). This overrode auto-detection, so repos with 90%+ tests in test/ or test/specs/ got told to co-locate. The LLM received conflicting signals (prompt says co-locate, file tree shows separate dir) and picked randomly each run.