Add untitled file support to Copilot CLI prompt resolver

[Copilot]

The CLI wasn't handling untitled file references when attached as context, unlike the cloud sessions provider which reads and embeds their content.

Changes

• Separated untitled file handling in constructChatVariablesAndAttachments() to detect scheme === 'untitled' references
• Added extractUntitledFileContent() method to read untitled file content via vscode.workspace.openTextDocument() and wrap with <file-start>/<file-end> tags
• Updated resolvePrompt() to append untitled file context to the generated prompt
• Used dynamic import import('vscode') to satisfy ESLint restrictions (src/**/node/**/* paths cannot use runtime vscode imports)

Pattern

Matches copilotCloudSessionsProvider.ts (lines 978-989) implementation:

// Untitled files are now embedded in prompt context:
// The user has attached the following untitled files as relevant context:
// <file-start>/Untitled-1</file-start>
// [file content]
// <file-end>/Untitled-1</file-end>

Original prompt

for the CLI - copy how the cloud sessions attached file references. We need the CLI to get the untitled file

The user has attached the following file paths as relevant context:

• src/extension/chatSessions/vscode-node/copilotCloudSessionsProvider.ts
• .github/copilot-instructions.md

Created from VS Code.

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