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/stream_json_session.dart

@@ -794,6 +794,13 @@ class StreamJsonSession {
     _setBusy(true);
   }
 
+  /// Inject a clide-local notice card into the conversation — nothing is sent
+  /// to claude. Used by the slash-command router for TUI-only commands
+  /// (T-411); renders as the muted synthetic "clide" card.
+  void addLocalNotice(String text) {
+    _items.add(AssistantTextMessage(uuid: 'local-${_localSeq++}', timestamp: DateTime.now(), isSidechain: false, text: text, synthetic: true));
+  }
+
   /// Interrupt the running turn (the escape hatch for a runaway — D-78). Sends
   /// the `interrupt` control_request; claude cancels the current turn and ends
   /// it with a `result`, which clears [busy]. Safe to call when idle.