Use VS Code's worktree InputBox UI when checking out a PR in a worktree

[Copilot]

"Checkout in Worktree" opened the OS save dialog, which is awkward (especially on Windows) and inconsistent with VS Code's built-in Git: Create Worktree... flow that uses an inline InputBox.

Changes — src/github/worktree.ts

• Replace showSaveDialog with createInputBox in checkoutPRInWorktree, mirroring the built-in worktree UI: pre-filled editable path, prompt "Please provide a worktree path", ignoreFocusOut, and an inline folder-picker button (QuickInputButtonLocation.Inline, folder codicon) that opens showOpenDialog to pick a parent directory and re-joins the worktree name.
• Default path now follows the built-in convention <parentDir>/<repoBasename>.worktrees/pr-<number> (was <parentDir>/pr-<number>), matching what users see from Git: Create Worktree....
• Pre-select the trailing pr-<number> segment via valueSelection for quick rename.
• Warn on collision with an existing worktree using repository.state.worktrees and InputBoxValidationSeverity.Warning (non-blocking, matching built-in).

Public signature of checkoutPRInWorktree and the subsequent "open in current/new window" prompt are unchanged.

const worktreePath = await promptForWorktreePath(repositoryToUse, worktreeName, defaultWorktreePath);
if (!worktreePath) {
    return; // User cancelled
}
const worktreeUri = vscode.Uri.file(worktreePath);