feat(chat): /steer non-interruptive support (Phase 2.1)

Hermes v2026.4.23 introduces /steer — mid-run guidance the agent
applies after the next tool call without interrupting the current
turn. Surface it as a first-class slash command in both Mac and iOS
chat menus with non-interruptive send semantics.

ScarfCore RichChatViewModel:
- nonInterruptiveCommands static (currently just /steer) merged
  into availableCommands at the end of the menu.
- HermesSlashCommand.Source.acpNonInterruptive case carries the
  flag through to the menu UI.
- transientHint: String? property for short-lived composer toasts.
- isNonInterruptiveSlash(_ text: String) -> Bool helper for the
  send paths to detect /steer-shaped invocations.

Mac ChatViewModel.sendViaACP:
- /steer-shaped sends skip the "Agent working..." status update
  (the agent is already on its current turn) and set a 4-second
  transientHint "Guidance queued — applies after the next tool call."

Mac RichChatView:
- New steeringToast() above the input bar renders the hint when
  set; tinted pill with arrow icon, opacity transition.

iOS ChatController.send + ChatView:
- Same isNonInterruptiveSlash check surfaces the toast above the
  composer; auto-clears via the same 4s Task pattern.
- steeringToast() helper view in ChatView.

Verified: ScarfCore + Mac + iOS builds clean.

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

scarf/Scarf iOS/Chat/ChatView.swift

@@ -47,6 +47,9 @@ struct ChatView: View {
             projectContextBar
             messageList
             Divider()
+            if let hint = controller.vm.transientHint {
+                steeringToast(hint)
+            }
             composer
         }
         .navigationTitle("Chat")
@@ -242,6 +245,26 @@ struct ChatView: View {
     }
 
     @ViewBuilder
+    /// Soft pill above the composer confirming a non-interruptive
+    /// command was received (e.g. `/steer`). Auto-clears via the
+    /// 4-second Task in `ChatController.send()`.
+    private func steeringToast(_ hint: String) -> some View {
+        HStack(spacing: 6) {
+            Image(systemName: "arrowshape.turn.up.right.fill")
+                .foregroundStyle(.tint)
+                .font(.caption)
+            Text(hint)
+                .font(.caption)
+                .foregroundStyle(.primary)
+            Spacer(minLength: 0)
+        }
+        .padding(.horizontal, 12)
+        .padding(.vertical, 6)
+        .frame(maxWidth: .infinity, alignment: .leading)
+        .background(.tint.opacity(0.12))
+        .transition(.opacity)
+    }
+
     private var composer: some View {
         HStack(alignment: .bottom, spacing: 8) {
             TextField(
@@ -557,6 +580,19 @@ final class ChatController {
         guard !sessionId.isEmpty else { return }
         draft = ""
         vm.addUserMessage(text: text)
+        // /steer is non-interruptive — the agent is still on its
+        // current turn; the guidance applies after the next tool call.
+        // Surface a transient toast confirming the guidance was
+        // received. v2.5 / Hermes v2026.4.23+.
+        if vm.isNonInterruptiveSlash(text) {
+            vm.transientHint = "Guidance queued — applies after the next tool call."
+            Task { @MainActor [weak vm] in
+                try? await Task.sleep(nanoseconds: 4_000_000_000)
+                if vm?.transientHint == "Guidance queued — applies after the next tool call." {
+                    vm?.transientHint = nil
+                }
+            }
+        }
         // Project-scoped slash commands expand client-side: the user
         // bubble shows the literal `/<name> args` they typed (above);
         // Hermes receives the expanded prompt template body. Other