Add adk graph command and --ascii-only for DOT output

### Summary:
Add CLI command adk graph to render agent trees as DOT/PNG/SVG, plus an --ascii-only option to strip non-ASCII for Windows consoles.
Motivation: Improve developer experience to visualize multi-agent hierarchies without launching the web UI; enable Windows CMD users to generate readable DOT.
### Scope:
New subcommand: adk graph
Formats: dot|png|svg
Option: --ascii-only for DOT
### Acceptance Criteria:
Running adk graph on a sample agent produces DOT with readable labels.
Optionally produces PNG/SVG when Graphviz is installed.
Windows CMD can generate ASCII-only DOT without encoding errors.