#163 refactor: split iOS runner-client into focused modules

Summary

• Split the 1129-LOC runner-client.ts into 4 focused modules: runner-errors.ts, runner-xctestrun.ts, runner-transport.ts, runner-session.ts
• Slim runner-client.ts to a thin orchestration layer (~120 LOC) with re-exports preserving all public API
• Internal-only functions are properly un-exported to minimize surface area

Fixes #155

Test plan

• pnpm typecheck passes
• pnpm test:unit — 514/514 tests pass
• pnpm test:smoke — 7/7 tests pass
• All existing imports from runner-client.ts work unchanged (verified via grep)

🤖 Generated with Claude Code