feat: v0.8.0 edit and save files with conflict detection

- devdesk attach --fs-write SESSION PATH BYTES BASESHA|new: streamed like
  uploads, temp file in the same folder, sha re-check (conflict reply instead
  of overwriting), mode/owner kept, atomic mv; new files with folders, never
  outside the session folder or through a symlink out of it.
- Editor: editable CodeMirror for UTF-8 text, dirty tracking, Ctrl+S,
  Save/Revert, conflict dialog with a disk-vs-yours diff, changed-on-disk
  banner, drafts in localStorage (flushed on pagehide), close/leave guards,
  New file.
- Upload streaming factored into streamIn(), shared by uploads and saves.

Closes #20

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

bin/devdesk

@@ -318,7 +318,7 @@ cmd_attach() {
         dd_linger
         return
         ;;
-    --fs-list | --fs-find | --fs-read | --git-status | --git-show | --asset)
+    --fs-list | --fs-find | --fs-read | --fs-write | --git-status | --git-show | --asset)
         local action=${1#--}
         shift
         "cmd_${action//-/_}" "$@"