service-ai-visualize-data-tool.md

@objectstack/service-ai	minor

feat(service-ai): visualize_data tool — return charts from AI data queries

Adds a visualize_data AI tool so the data-query assistant can answer with a CHART instead of plain text/markdown. The tool runs an aggregation through the existing analytics service (auto-inferred cube), maps the result into the SDUI <chart> contract, and emits it to the client as a data-chart custom stream part (the same onProgress channel data-build-progress already uses). It also returns a compact textual summary so the model narrates the answer alongside the rendered chart.

• tools/visualize-data.tool.ts — tool definition, handler, and register fn (function+field → analytics measure key; single dimension → x-axis; measures → series; chartType bar/line/pie/…).
• plugin.ts — registers the tool when an analytics service is present and persists it as tool metadata in lockstep (Studio visibility).
• skills/data-explorer-skill.ts — exposes visualize_data plus chart trigger phrases and guidance to prefer it for "chart/plot/trend/breakdown" requests.