show the configured skills count in the Claude status line

The status slot showed only live session fields (model/mode/context).
It now also shows the configured skills count from ClaudeConfig — the
environment side alongside the live session — and the pane rebuilds when
the config changes so the count appears once skills load and tracks
.claude edits.

T-154.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/builtin/claude/claude_status_test.dart

@@ -26,6 +26,12 @@ void main() {
       expect(formatTokenCount(765000), '765k');
       expect(formatTokenCount(1200000), '1.2M');
     });
+
+    test('formatSkillsLabel pluralises and hides zero', () {
+      expect(formatSkillsLabel(0), isNull);
+      expect(formatSkillsLabel(1), '1 skill');
+      expect(formatSkillsLabel(12), '12 skills');
+    });
   });
 
   group('formatStatusLine', () {