feat: files pane like VS Code: tree, tabs, go to file

- Explorer tree with inline expand/collapse and keyboard navigation, git
  colours, change dots, dimmed gitignored entries (fs-list now flags them),
  file-type badges, resizable explorer.
- Editor tabs: preview tab semantics, instant switching with scroll kept,
  in-place reload when Claude changes an open file.
- Go to file: devdesk attach --fs-find (git ls-files or capped find) and a
  fuzzy-ranked palette (CommandPalette gained custom search/async lists).
- Per-session memory of folders and tabs; empty state with recents.

Closes #37

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

bin/devdesk

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