Experimental: Add experimental built-in visualization tools to Gemini CLI

## Summary
Gemini CLI needs a built-in visualization capability for terminal-native charts/diagrams so users can ask for visual comparisons and trends in natural language without leaving the CLI.

## Problem
Today, visual outputs are inconsistent and model calls can require schema retries. This breaks flow for prompts like:
- "research the fastest 3 BMW and show me a chart of 0-60"
- follow-up questions in the same session that require a different visualization shape

## Proposal
Ship built-in visualization tools in Gemini CLI as an experimental feature, with clear model steering hints so tool calls succeed on the first attempt more often.

### Scope
- Native tool-driven visualizations in terminal output
- Canonical payload shapes + shorthand aliases for robust model input
- Prompt/tool-description steering guidance for kind selection and payload formatting
- Diagram rendering that is direction-aware and visually structured

## Acceptance Criteria
- Model can select a visualization kind from natural-language requests without user-provided schema
- First-shot tool call success rate improves materially vs current baseline
- Follow-up turns in the same session can render different visualization kinds correctly
- Feature is clearly marked experimental in user-facing docs/PR notes

## Why this matters
This improves comprehension and trust during reasoning loops by giving users compact, visual outputs directly in the CLI.