Boundary: Canonical, reusable 4-phase RCA engine. Invoked by investigate-bug (as step 2 of the end-to-end flow) and by fix-bug (when no bug file exists). Does not write the bug file — that is investigate-bug’s responsibility.

Four phases — do not skip. Update the active specs/bugs/BUG-*.md file at each phase.

1. Reproduce — minimal steps; record environment; capture logs.
2. Isolate — narrow to module/function; binary-search commits or config.
3. Hypothesize — list ranked hypotheses with falsification test each.
4. Verify — run falsification; confirm single root cause; link to fix plan.

HARD GATE — Do not propose a fix until phase 4 confirms one root cause with evidence.

→ verify: [ "$(grep -lE 'Reproduce' specs/bugs/BUG-*.md 2>/dev/null | grep -cv "\.okf\.md$")" -ge 1 ]