- Move formatChildExitSummary/formatCapturedStderrTail to worker-runtime.ts
- Move ignoreSessionBroadcastSignals to new worker-signals.ts
- Delete worker-fork-support.ts (teeCapturedStderr no longer used)
- Update .knowledge/worker-isolation.md and architecture.md for WorkerRuntime
- All 167 tests pass, biome check clean
Closes#283