route slash commands: TUI-only builtins become local notices (T-411)

clide forwards composer input to a headless (stream-json) CLI, where the
TUI's interactive commands don't exist. A known-but-TUI-only command
errored raw ("/x isn't available in this environment", rendered as fake
claude prose); an un-advertised one (e.g. /effort on 2.1.175) was worse —
bracket-pasted to the model as literal text, burning a real turn.

Probed claude 2.1.175 for ground truth: the initialize handshake's
slash_commands advertises skills + the headless builtins only; forwarded
local-command output comes back as an assistant message with model
"<synthetic>"; set_effort is not a control subtype; /usage works headless.

- slash_commands.dart: SlashRoute routing table (owned > advertised >
  TUI-only catalog > forward) + kTuiOnlyCommands with clide-native hints
  + tuiOnlyNotice(). One source of truth replacing ad-hoc checks.
- claude_pane._send routes 'unavailable' to a local notice card; nothing
  reaches the session.
- transcript_reader: AssistantTextMessage.synthetic ("<synthetic>" model)
  so CLI-local output is distinguishable; "<synthetic>" no longer
  clobbers the tracked model in SessionStatus (latent /usage bug).
- conversation_view: synthetic output renders as a muted framed "clide"
  card (T-306 styling), never coral Claude prose.
- kFallbackSlashCommands trimmed to the genuinely-headless builtin set —
  it doubles as the router's advertised fallback, and the old list's
  TUI-only entries would have routed to a raw CLI error.

Board (rides this commit): T-414 gains the user's sidebar styling-pass
note; T-416 filed — surface Claude Code Workflow runs in convo/status.

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

test/builtin/claude/transcript_reader_test.dart

@@ -524,6 +524,51 @@ void main() {
     });
   });
 
+  group('synthetic CLI-local output (T-411)', () {
+    String syntheticEvent(String text) => jsonEncode({
+      'type': 'assistant',
+      'uuid': 'syn1',
+      'timestamp': '2026-06-12T10:00:00.000Z',
+      'message': {
+        'role': 'assistant',
+        'model': '<synthetic>',
+        'content': [
+          {'type': 'text', 'text': text},
+        ],
+      },
+    });
+
+    test('a "<synthetic>" assistant message parses with synthetic: true', () {
+      final parsed = parseTranscriptChunk(syntheticEvent("/effort isn't available in this environment."));
+      final msg = parsed.items.whereType<AssistantTextMessage>().single;
+      expect(msg.synthetic, isTrue);
+      expect(msg.text, contains("isn't available"));
+    });
+
+    test('"<synthetic>" never clobbers the tracked model', () {
+      final parsed = parseTranscriptChunk(syntheticEvent('usage text'));
+      expect(parsed.status.model, isNull);
+    });
+
+    test('a real assistant message stays non-synthetic', () {
+      final chunk = jsonEncode({
+        'type': 'assistant',
+        'uuid': 'a2',
+        'timestamp': '2026-06-12T10:00:00.000Z',
+        'message': {
+          'role': 'assistant',
+          'model': 'claude-fable-5',
+          'content': [
+            {'type': 'text', 'text': 'hello'},
+          ],
+        },
+      });
+      final parsed = parseTranscriptChunk(chunk);
+      expect(parsed.items.whereType<AssistantTextMessage>().single.synthetic, isFalse);
+      expect(parsed.status.model, 'claude-fable-5');
+    });
+  });
+
   group('TranscriptReader — append streaming (filesystem)', () {
     late Directory tempBase;