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/editor/index.js

@@ -4,7 +4,7 @@
 // the first time the Files pane opens, so the page itself stays small.
 import { EditorView, keymap, lineNumbers, highlightActiveLine, highlightActiveLineGutter, drawSelection, highlightSpecialChars } from '@codemirror/view';
 import { EditorState } from '@codemirror/state';
-import { defaultKeymap, history, historyKeymap } from '@codemirror/commands';
+import { defaultKeymap, history, historyKeymap, indentWithTab } from '@codemirror/commands';
 import { syntaxHighlighting, HighlightStyle, StreamLanguage, bracketMatching, foldGutter, foldKeymap } from '@codemirror/language';
 import { search, searchKeymap, highlightSelectionMatches } from '@codemirror/search';
 import { markdownLanguage } from '@codemirror/lang-markdown';
@@ -135,12 +135,40 @@ function baseExtensions(path, { readOnly = true, wrap = false } = {}) {
   ];
 }
 
-// Read-only (v1) view of a file.
+// Read-only view of a file.
 export function viewer(parent, { doc, path, wrap }) {
   const view = new EditorView({ parent, state: EditorState.create({ doc, extensions: baseExtensions(path, { wrap }) }) });
   return { view, destroy: () => view.destroy() };
 }
 
+// Editable file (devdesk#20): onChange(text) after every edit, onSave() on
+// Ctrl/Cmd+S (which never reaches the browser or the terminal).
+export function editor(parent, { doc, path, wrap, onChange, onSave }) {
+  const view = new EditorView({
+    parent,
+    state: EditorState.create({
+      doc,
+      extensions: [
+        keymap.of([{ key: 'Mod-s', preventDefault: true, run: () => { onSave?.(); return true; } }, indentWithTab]),
+        baseExtensions(path, { readOnly: false, wrap }),
+        EditorView.updateListener.of((update) => {
+          if (update.docChanged) onChange?.(update.state.doc.toString());
+        }),
+      ],
+    }),
+  });
+  return {
+    view,
+    text: () => view.state.doc.toString(),
+    // Replace the whole document (reload from disk) keeping the cursor roughly where it was.
+    setText: (text) => {
+      const pos = Math.min(view.state.selection.main.head, text.length);
+      view.dispatch({ changes: { from: 0, to: view.state.doc.length, insert: text }, selection: { anchor: pos } });
+    },
+    destroy: () => view.destroy(),
+  };
+}
+
 // Unified diff of the HEAD version against the working copy.
 export function diff(parent, { original, modified, path, wrap }) {
   const view = new EditorView({