Add isCopilot telemetry property to pr.checkout events

[Copilot]

Track whether checked-out PRs are authored by Copilot, matching existing behavior in pr.openDescription events.

Changes

• src/view/reviewsManager.ts: Added isCopilot property to pr.checkout telemetry alongside existing fromDescription property
• src/view/reviewManager.ts: Added isCopilot property to pr.checkout telemetry in the switch method
• Both locations now check author.login === COPILOT_SWE_AGENT and send boolean as string ('true'/'false')
• Updated GDPR telemetry comments to document the new property
• Fixed pre-existing GDPR comment bug: fromDescriptionPage → fromDescription to match actual property name

Implementation

// Pattern used (matching pr.openDescription):
this._telemetry.sendTelemetryEvent('pr.checkout', { 
  isCopilot: (pr.author.login === COPILOT_SWE_AGENT) ? 'true' : 'false' 
});

Original prompt

Just like we do with the 'pr.openDescription event, add an 'isCopilot' property for pr.checkout telemetry to track if the checked out branch is a copilot branch (we should be able to tell by the pr author)

The user has attached the following files as relevant context:

• src/github/pullRequestOverview.ts
• .github/copilot-instructions.md

Created from VS Code via the GitHub Pull Request extension.

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