Adopt codicon tree item label for current issue in issues view

[Copilot]

The issues view currently uses a text checkmark ✓ to indicate the current issue, while the pull requests view uses the $(check) codicon. This creates visual inconsistency.

Changes

• Replace text ✓ with $(check) codicon for current issue indicator
• Wrap label in vscode.MarkdownString with supportThemeIcons: true to enable codicon rendering
• Escape $(...) patterns in issue titles to prevent unintended icon rendering

Implementation

// Before
treeItem.label = `✓ ${treeItem.label as string}`;

// After
const escapedTitle = element.title.replace(/\$\([a-zA-Z0-9~-]+\)/g, '\\$&');
const label: vscode.TreeItemLabel2 = {
    label: new vscode.MarkdownString(`$(check) ${escapedTitle}`, true)
};
treeItem.label = label as vscode.TreeItemLabel;

Matches the existing pattern in pullRequestNode.ts (lines 306, 322, 336-347).

Original prompt

Adopt the codicon tree item label in the issues view for the current issue.

Created from VS Code via the GitHub Pull Request extension.

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