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>

lib/builtin/claude/src/transcript_reader.dart

@@ -114,12 +114,19 @@ final class AssistantTextMessage extends ConversationItem {
     super.parentUuid,
     super.parentToolUseId,
     required this.text,
+    this.synthetic = false,
   });
 
   final String text;
 
+  /// CLI-local output, not the model: the wire marks it `model: "<synthetic>"`
+  /// (a forwarded local command's response — /usage output, "/x isn't
+  /// available in this environment", …). clide-injected notices use it too.
+  /// Rendered as a muted "clide" card, never coral Claude prose (T-411).
+  final bool synthetic;
+
   @override
-  String toString() => 'AssistantTextMessage(${_shortId(uuid)}, ${text.length} chars)';
+  String toString() => 'AssistantTextMessage(${_shortId(uuid)}, ${text.length} chars${synthetic ? ', synthetic' : ''})';
 }
 
 /// Extended thinking block from an assistant turn.
@@ -582,7 +589,9 @@ void _extractAssistantStatus(Map<String, dynamic> envelope, _StatusAcc status) {
   final message = envelope['message'] as Map?;
   if (message == null) return;
   final model = message['model'] as String?;
-  if (model != null && model.isNotEmpty) status.model = model;
+  // "<synthetic>" marks CLI-local output (a forwarded local command's
+  // response) — not a model switch; it must not clobber the tracked model.
+  if (model != null && model.isNotEmpty && model != kSyntheticModel) status.model = model;
   final usage = message['usage'] as Map?;
   if (usage != null) {
     int n(String k) => (usage[k] as num?)?.toInt() ?? 0;
@@ -656,6 +665,9 @@ void _parseUserInto(
   }
 }
 
+/// The model marker on CLI-local output (forwarded local-command responses).
+const String kSyntheticModel = '<synthetic>';
+
 void _parseAssistantInto(
   Map<String, dynamic> envelope,
   String uuid,
@@ -669,6 +681,7 @@ void _parseAssistantInto(
   if (message == null) return;
   final content = message['content'];
   if (content is! List) return;
+  final synthetic = (message['model'] as String?) == kSyntheticModel;
 
   for (final item in content) {
     if (item is! Map) continue;
@@ -684,6 +697,7 @@ void _parseAssistantInto(
               parentUuid: parentUuid,
               parentToolUseId: parentToolUseId,
               text: text,
+              synthetic: synthetic,
             ),
           );
         }