refactor: unify all command functions as async generators

All command functions now use async generator signatures. The framework
iterates each yielded value through the existing OutputConfig rendering
pipeline. Non-streaming commands yield once and return; streaming
commands (log list --follow) yield multiple times.

Key changes:
- buildCommand: func returns AsyncGenerator, wrapper uses for-await-of
- All ~27 command files: async func → async *func, return → yield
- log/list follow mode: drainStreamingOutput replaced by yield delegation
- Delete streaming-command.ts (151 lines) — absorbed into buildCommand
- output.ts: extracted applyJsonExclude/writeTransformedJson helpers,
  support undefined suppression for streaming text-only chunks

No new dependencies. Net -131 lines.

Author: BYK