add multi-file editor tabs

The editor pane showed a single buffer — opening a second file
replaced the first, even though the daemon's EditorRegistry has
always been multi-buffer (editor.list / activate / close). This wires
the UI up to that: EditorController now tracks the full open-buffer
list (via editor.list on hydrate, kept in sync by editor.opened /
closed / saved / edited events), and EditorView renders the buffers
as tabs through the shared MultitabPane — the same strip the Claude
pane uses. The daemon stays the source of truth: the local tab
controller is reconciled from it, and tab select / close route back
as editor.activate / editor.close. Reorder is disabled for now
(daemon order is authoritative).

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

CHANGELOG.md

@@ -18,6 +18,11 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- Multi-file editor tabs — the editor pane now shows one tab per open
+  buffer (filename + a dot when unsaved) via the shared tab strip;
+  opening a second file no longer replaces the first. Click a tab to
+  switch, × to close. Backed by the daemon's existing multi-buffer
+  model.
 - Typed IPC command-schema framework (T-119/T-120, D-74) — commands
   register an argument schema beside their handler; the dispatcher
   normalises argv into named args, coerces types, and validates