feat: v0.7.0 files pane: browse, preview, diff, Claude's edits

- devdesk attach --fs-list/--fs-read/--git-status/--git-show/--asset
  (libexec/devdesk-files.sh), confined to the session folder.
- The state hook records files Claude edits (Edit/Write/MultiEdit/
  NotebookEdit) for the Changes tab's "C" badge; rename/end carry it.
- Files pane: folders, CodeMirror read-only view, sanitized markdown preview
  in a sandboxed iframe with inlined relative images, images, Changes tab with
  line counts and diff against HEAD, "→ Claude" @path, resizable split.
- The editor bundle ships as /usr/share/devdesk/assets/editor.js, is fetched
  over ttyd on first use, verified against the page's SHA-256 and cached in
  IndexedDB.
- Fix: the state hook's field split collapsed empty fields.

Closes #19

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

debian/copyright

@@ -35,6 +35,11 @@ Comment: Bundled into index.html at build time:
  2014-2016 SourceLair Private Company, 2012-2013 Christopher Jeffrey);
  JetBrains Mono via @fontsource/jetbrains-mono (OFL-1.1,
  Copyright 2020 The JetBrains Mono Project Authors).
+ Bundled into assets/editor.js (the file viewer) at build time:
+ CodeMirror 6 and @lezer/highlight (MIT, Copyright 2018-2021 Marijn Haverbeke
+ and others); marked (MIT, Copyright 2018+ MarkedJS, 2011-2018 Christopher
+ Jeffrey); DOMPurify (Apache-2.0 or MPL-2.0 at the recipient's choice, here
+ Apache-2.0, Copyright 2025 Dr.-Ing. Mario Heiderich, Cure53).
 
 License: MIT
  Permission is hereby granted, free of charge, to any person obtaining a copy