fix decision first-click + editor reveal via retained reader nav

Two reveal-on-open bugs:

Decisions opened only on the second click (T-196): the detail view
subscribed in didChangeDependencies, which runs after the tab is
revealed, so the broadcast 'selection' that triggered the reveal was
already gone. Hoist the back/forward history out of per-view State into
a retained per-reader ReaderNav (kernel ChangeNotifier in a
ReaderNavRegistry, D-81). The nav records selections, emits 'load' (the
single channel readers display from), and survives mount/unmount — the
reader grabs nav.current on mount, so the first selection lands. Both
the markdown and decisions readers move to this model; the per-view
ReaderHistoryMixin and the markdown post-frame forward hack are gone.

The editor pane never opened (T-197): EditorExtension contributed a
workspace tab but nothing activated it on editor.open. Add an activate()
that reveals the tab on editor.opened / editor.active-changed; the
view's hydrate() pulls the active buffer on mount.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -46,6 +46,14 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- The editor pane now opens over the Claude pane when a file is opened — the
+  reader's edit pencil, a file-tree click, or a decision's edit all reveal the
+  editor tab now (it was contributed but never activated). (T-197)
+- Clicking a decision opens it on the first click. The right-pane readers
+  (markdown + decisions) now share a retained back/forward nav history that
+  survives the tab switch, so the selection that reveals a reader is no longer
+  lost before the widget subscribes; back/forward re-emit through that history.
+  (T-196)
 - The markdown reader can now open user-scope Claude config files (skills /
   agents / commands under `~/.claude`), not just repo-local ones. `files.read`
   gained a read allow-list covering the workspace plus the trusted Claude config