feat: add extension points API for companion extension button registration

[rajbos]

Summary

Introduces a generic extension points API that allows companion VS Code extensions to contribute buttons directly into the navigation toolbar of all webview panels — without needing a separate VS Code status bar item.

Changes

New files

• \src/extensionPoints.ts\ — Public API types (\ExtensionPointButton, \AiFluencyExtensionApi) exported from the extension's \�ctivate()\
• \src/webview/shared/extensionPoints.ts\ — Browser-side \wireExtensionPointButtons()\ helper that appends registered buttons to the .button-row\ and wires click events back to the host

Modified

• \src/extension.ts\
  • _extensionPointButtons\ registry +
    egisterExtensionPointButton()\ method
  • \�xtensionPointButtonsScript()\ injects \window.EXTENSION_POINT_BUTTONS\ into all 9 webview HTML generators
  • \handleExtensionPointAction()\ routes button clicks to registered handlers
  • \dispatchSharedCommand()\ now accepts the full message object and handles \�xtensionPointAction\ alongside existing commands
  • \�ctivate()\ returns \AiFluencyExtensionApi\
• All 8 webview panel \main.ts\ files (dashboard, details, chart, environmental, usage, maturity, fluency-level-viewer, diagnostics): import + call \wireExtensionPointButtons(vscode)\ after the button row is rendered

How it works

1. A companion extension calls \�pi.registerButton({ id, label }, handler)\ using the main extension's exported API
2. The button is injected into the HTML of every webview panel via \window.EXTENSION_POINT_BUTTONS\
3. The webview renders the button in the toolbar and posts \�xtensionPointAction\ on click
4. The main extension routes the click back to the registered handler