Activity tab: session controls + live usage block (T-415)

The Activity tab gains the power-panel's session strip and a usage block:

- SESSION controls (clear / compact / fork / resume + refresh-usage)
  publish their slash command on builtin.claude/command — the same path
  as typing it, so /clear semantics (and any future confirm behavior)
  live in exactly one place.
- The usage block revisits T-158's "blocked on upstream": probed against
  claude 2.1.175, a forwarded /usage IS answered headless, free
  (num_turns 0), as parseable text. parseUsageText() extracts session /
  week / week-Sonnet percentages (timezone parentheticals stripped); the
  sidebar watches the primary session's synthetic output for
  usage-shaped responses and renders them as a USAGE section. Refresh is
  user-initiated (the control sends /usage) — no polling, no background
  calls (D-64).
- The runtime row gains the session's effort level (T-412's status
  field).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/builtin/claude/src/meta_sidebar/models.dart

@@ -44,7 +44,12 @@ Widget metaPlaceholder(String text) => Padding(
 );
 
 /// Key→value sections on the shared table geometry (Activity + Config).
-Widget buildMetaTable(SurfaceTokens tokens, List<MetaSection> sections) {
+Widget buildMetaTable(SurfaceTokens tokens, List<MetaSection> sections) =>
+    ListView(padding: const EdgeInsets.all(12), children: metaTableChildren(tokens, sections));
+
+/// The table rows without the enclosing ListView, for tabs that compose extra
+/// widgets around the sections (the Activity tab's control strip, T-415).
+List<Widget> metaTableChildren(SurfaceTokens tokens, List<MetaSection> sections) {
   final children = <Widget>[];
   for (var i = 0; i < sections.length; i++) {
     final s = sections[i];
@@ -74,5 +79,5 @@ Widget buildMetaTable(SurfaceTokens tokens, List<MetaSection> sections) {
       );
     }
   }
-  return ListView(padding: const EdgeInsets.all(12), children: children);
+  return children;
 }