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>

tests/attach.bats

@@ -594,3 +594,23 @@ blob_body() {
         [ "$status" -ne 0 ]
     done
 }
+
+@test "fs-list flags gitignored entries; fs-find lists files for go-to-file (#37)" {
+    p="$DEVDESK_PROJECTS_ROOT/alpha"
+    git -C "$p" init -q -b main
+    mkdir -p "$p/src" "$p/build" "$p/node_modules/x"
+    printf 'build/\n*.log\n' >"$p/.gitignore"
+    echo a >"$p/src/a.js"
+    echo b >"$p/debug.log"
+    echo c >"$p/node_modules/x/i.js"
+    run devdesk attach --fs-list alpha
+    [ "$(jq -r '[.entries[] | select(.ig) | .n] | join(",")' <<<"$output")" = "build,debug.log" ]
+    run devdesk attach --fs-find alpha
+    [ "$(jq -r '.files | sort | join(",")' <<<"$output")" = ".gitignore,node_modules/x/i.js,src/a.js" ]
+    # Outside git: find, without node_modules
+    rm -rf "$p/.git"
+    run devdesk attach --fs-find alpha
+    [ "$(jq -r '.files | sort | join(",")' <<<"$output")" = ".gitignore,debug.log,src/a.js" ]
+    run env DEVDESK_FS_FIND_MAX=1 "$REPO/bin/devdesk" attach --fs-find alpha
+    [ "$(jq -r '[(.files | length), .truncated] | @tsv' <<<"$output")" = "$(printf '1\ttrue')" ]
+}