feat(files): git decorations in explorer and editor gutter

[SobshDev]

Summary

• Color file/folder rows in the explorer by working-tree git status (green = added/untracked, amber = modified, red = deleted). Folders inherit the worst status of their descendants.
• Add a CodeMirror 6 gutter to the file editor that paints per-line markers (green/amber/red) using git diff HEAD -- <file>. Refreshes on file open, save, and project change. Untracked files render as all-added.
• Extend the graphy:git:status IPC to return modified / deleted / renamed separately and graphy:git:diff to accept a ref (so the editor can ask for working-tree vs HEAD).

Test plan

• Open a repo and edit a tracked file. The file row in the explorer turns amber; on save, the corresponding lines in the editor get an amber gutter bar.
• Create a new file. The file row turns green and every line in the editor has a green bar.
• Delete a tracked file via the OS or git rm. The file row turns red.
• In a single edited file, delete a block of lines. After saving, a red gradient marker appears on the line that follows the deletion.
• Folder colors reflect the worst descendant status (deleted > modified > added).
• Source Control panel still lists changed files correctly (verifies the IPC shape change didn't regress useDiffChanges).
• Switch between files — gutter markers update without stale data from the previously open file.