Register Slack charts as Channel components

[tylerslaton]

Summary

• convert render_chart from a manually posted defineChannelTool to defineChannelComponent
• register RenderChart through createChannel({ components })
• remove the duplicate manual entry from appTools
• preserve native Slack.Block.DataVisualization rendering on Slack and render a portable unsupported-platform notice elsewhere
• add a Channel-level integration test proving an agent component call posts the native Slack visualization block

Why

The initial visualization PR used the new Slack block but did not use the Channels SDK's agent-rendered component abstraction. This follow-up makes render_chart a first-class Channel component so the SDK owns argument validation, posting, and tool exposure.

Impact

The agent-facing name and chart schema remain render_chart; the registration path now uses the component registry as intended.

Validation

• pnpm install --frozen-lockfile
• pnpm check-types
• pnpm test — 160 passed
• cd agent && uv run pytest — 70 passed
• node node_modules/railway/dist/iac/bin.js
• git diff --check