Use native Slack data visualization blocks

[tylerslaton]

Summary

• upgrade @copilotkit/channels to 0.7.0 and the matching @copilotkit/runtime to 1.66.0
• replace the headless Chart.js PNG/upload path with Slack.Block.DataVisualization
• support Slack-native pie, bar, area, and line payloads with provider-limit and cross-field validation
• migrate SDK identity-sensitive paths from the removed PlatformUser ingress shape to Channels 0.7's ProviderActor
• update tests and pinned-version documentation

Why

Channels 0.7 ships Slack's native data visualization component. Using it removes the Chart.js CDN/render/upload hop and sends charts as first-class Slack Block Kit payloads.

Impact

Slack chart requests now render natively in the thread. render_chart returns an explicit unsupported-platform result outside Slack. Mermaid diagrams continue to use the shared Playwright renderer.

Validation

• pnpm install --frozen-lockfile
• pnpm check-types
• pnpm test — 159 passed
• cd agent && uv run pytest — 70 passed
• node node_modules/railway/dist/iac/bin.js
• git diff --check origin/main...HEAD