VS Code-compatible editors: optional clean obsolete extension directories

## Checklist - [x] I have searched the issue tracker for relevant or duplicate issues. VS Code-compatible editors record obsolete extension directories in `.obsolete` files under their extension roots: ```text ~/.cursor/extensions/.obsolete ~/.vscode/extensions/.obsolete ~/.vscode-insiders/extensions/.obsolete ``` Example: ```text obsolete entry: anthropic.claude-code-2.1.142-darwin-arm64 active manifest: ~/.cursor/extensions/anthropic.claude-code-2.1.143-darwin-arm64 ``` Would Topgrade accept an opt-in cleanup after successful VS Code-compatible extension updates? Requested behavior: 1. For each VS Code-compatible editor variant Topgrade updates, find that variant's extension root. 2. Read that root's `.obsolete` file, if present. 3. Delete only existing direct child directories named by that file. 4. Leave `.obsolete` itself intact. 5. Skip or warn if the relevant editor is running. Topgrade currently appears to have a shared `[vscode]` config for VS Code-compatible editors, not a Cursor-specific config section. Possible new config shape: ```toml [vscode] cleanup_obsolete_extensions = true ``` or per variant: ```toml [cursor] cleanup_obsolete_extensions = true [vscode_insiders] cleanup_obsolete_extensions = true ``` Alternatively, this could be gated behind Topgrade's existing global cleanup flag: ```toml [misc] cleanup = true ``` Safety: use `.obsolete` as the editor's source of truth rather than sorting versions, reject path traversal, and only delete direct children of the relevant extension root. <!-- Assuming that someone else implements the feature, please state if you know how to test it from a side branch of Topgrade. --> - [x] I am able and willing to implement this feature myself