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>

web/index.template.html

@@ -96,6 +96,7 @@
       <button type="button" class="dd-icon-btn dd-explorer-toggle" title="Show / hide the explorer" aria-label="Toggle explorer">☰</button>
       <div class="dd-tabs" role="tablist" aria-label="Open files"></div>
       <span class="dd-files-actions">
+        <button type="button" class="dd-icon-btn dd-files-new" title="New file" aria-label="New file">＋</button>
         <button type="button" class="dd-icon-btn dd-files-find" title="Go to file (Ctrl+P)" aria-label="Go to file">⌕</button>
         <button type="button" class="dd-icon-btn dd-files-refresh" title="Refresh" aria-label="Refresh">↻</button>
         <button type="button" class="dd-icon-btn dd-files-close" title="Close (Esc)" aria-label="Close files">✕</button>