Add gear icon to Description label that opens pullRequestDescription setting

[Copilot]

Adds a settings gear icon next to the "DESCRIPTION" label in the create PR view to make the githubPullRequests.pullRequestDescription setting more discoverable.

Changes

Webview UI (webviews/createPullRequestViewNew/app.tsx)

• Added gear icon button positioned right of "DESCRIPTION" label
• Handler posts pr.openDescriptionSettings message on click/Enter/Space
• Used semantic <button> with aria-label for accessibility

Extension (src/github/createPRViewProvider.ts)

• Added message handler that executes workbench.action.openSettings with the setting name

Styling (webviews/createPullRequestViewNew/index.css)

• Added group-description-header flex container for label + icon
• Icon at 70% opacity, 100% on hover/focus with smooth transitions

Screenshot

The gear icon opens VS Code settings focused on githubPullRequests.pullRequestDescription, following the same pattern as the sparkle icon on the Title field.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make githubPullRequests.pullRequestDescription more discoverable</issue_title>
<issue_description>We could have something in the Create webview to suggest this setting

cc @hediet </issue_description>

<agent_instructions>Add a gear icon button to the right of "DESCRIPTION" in the create view, which opens the settings editor with the setting.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@alexr00 Can be addressed in conjunciton with https://github.com//issues/4403 to prevent too many changes to the create view.

• Fixes Make githubPullRequests.pullRequestDescription more discoverable #4307

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