Add verbosity levels to agent CLI

Summary

Add verbosity levels for better debugging of agent runs.

Levels

| Level | Flag | Shows | |-------|------|-------| | 0 | (default) | Final output only | | 1 | -v | Tool calls + errors | | 2 | -vv | + Model thinking/reasoning | | 3 | -vvv | + Full API requests/responses |

Implementation

• Count -v flags or use --verbose=N
• Pass verbosity level to engine callbacks
• engineOnAssistant callback exists but isn't wired to CLI output