[Feature]: Add "Add to .gitignore" option to File Explorer context menu

## Problem or use case

The File Explorer context menu currently offers actions like Copy, Copy Path, Rename, and Delete — but no way to quickly add a file or folder to `.gitignore` without leaving the editor.

When working with generated files, build outputs, or secrets that accidentally land in the worktree, the typical workflow is: open `.gitignore`, find the right section, type the path, save. This is friction that every other major editor (VS Code, JetBrains IDEs, Xcode) eliminates with a single context menu action.

## Proposed solution

Add an **"Add to .gitignore"** entry to the File Explorer right-click context menu, available for both files and directories.

Behavior:
- Appends the relative path of the selected item to the project root's `.gitignore` (creates the file if it does not exist).
- For directories, appends with a trailing `/`.
- If the path is already present in `.gitignore`, show a brief notice instead of adding a duplicate.
- Only show the entry for items inside a git repository (not plain folders).
- Refresh the File Explorer decoration state after writing so gitignored styling applies immediately (related: #1941, #2021).

## Screenshot — current context menu (missing the action)


## Alternatives considered

Manually editing `.gitignore` or using the terminal (`echo "path" >> .gitignore`). Both work but break flow inside the IDE.

Start by locating the File Explorer context-menu entry point and the existing git decoration refresh flow. Trace how the selected item's relative path and repository root are determined, then inspect related issues #1941 and #2021. Done means files and directories can be added once to the repository .gitignore, only inside git repositories, with immediate decoration refresh.

Written by the indexing model from the issue text.