Copy path / reveal-in-Explorer context actions

[Wikid82]

Problem

The bookmark context menu only offers Remove and Move to Folder. Small but common conveniences from VS Code's native file Explorer context menu are missing for bookmarked items: copying the path, or revealing the file in the OS file manager / VS Code's Explorer view.

Proposal

Add to the bookmark item context menu:

• "Copy Path" (absolute) and "Copy Relative Path" — reuse `bookmark.relativePath` / `vscode.Uri.parse(bookmark.uri).fsPath`.
• "Reveal in Explorer" — `vscode.commands.executeCommand('revealInExplorer', uri)`.
• "Reveal in File Manager" — `vscode.commands.executeCommand('revealFileInOS', uri)`.

Low-effort, high day-to-day value — good first issue.