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/stream_json_session_test.dart

@@ -640,6 +640,16 @@ void main() {
     expect(statuses.last.permissionMode, 'plan', reason: 'only ExitPlanMode exits plan mode');
   });
 
+  test('addLocalNotice emits a synthetic clide item and sends nothing (T-411)', () async {
+    final before = proc.writes.length;
+    session.addLocalNotice('/status is a Claude Code TUI command');
+    await Future<void>.delayed(Duration.zero);
+    final notice = items.whereType<AssistantTextMessage>().single;
+    expect(notice.synthetic, isTrue);
+    expect(notice.text, contains('/status'));
+    expect(proc.writes.length, before); // nothing went to the CLI
+  });
+
   test('resolvePrompt(deny) writes a deny decision with a message', () async {
     proc.emit(canUseTool('req-3'));
     await Future<void>.delayed(Duration.zero);