feat(workflows): expose transport-agnostic flow executor

[tomatotomata]

Summary

• expose a reusable executeFlow primitive from core for CLI, SDK, scheduler, and voice adapters
• thread each step's output into the next step
• support __approval_gate__ through an injected callback with explicit rejected status
• emit transport-neutral progress events and add focused coverage

This addresses the missing core execution loop described in #89 without coupling the workflow module to a specific transport or agent runtime.

Verification

• npm run build
• npm test (68 tests)
• node --test test/workflows.test.ts --experimental-strip-types (3 tests)
• git diff --check

[tomatotomata]

Closing as superseded by the existing upstream PR #90, which covers the same SkillFlow execution surface. The implementation is retained locally for reference.