feat: queue prompts while the agent is running (#598)

Allow the chat input to be sent while the agent is mid-turn or awaiting
approval. The backend's prompt.submit busy-input policy already queues any
prompt that lands during a running turn (tui_gateway/server.py:_handle_busy_submit)
and drains it as the next turn, so the message is never dropped.

- ChatInputPolicy: pure, testable send/placeholder decision logic
- ChatScreen: canSend stays enabled while typing/approving; placeholder shows
  'Queued — sends when agent is free' when text is present during a busy turn
- ChatInputPolicyTest: truth-table covering typing, approval-pending,
  disconnected, empty, attachment, and slash-command cases
- strings: chat_input_placeholder_queue

Note: this supersedes the Genesi599 fork's approach (dispatchViaRpc('queue ...')),
which double-submits and relies on a 4018-prone command.dispatch path. The
backend-queued prompt.submit path is the correct, single-submission contract.

Co-authored-by: M57 <m57@hermes-control.app>

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatInputPolicy.kt

@@ -0,0 +1,35 @@
+package com.m57.hermescontrol.ui.chat
+
+/**
+ * Pure, testable decision logic for the chat input bar.
+ *
+ * The gateway's `prompt.submit` busy-input policy queues any prompt that lands
+ * while the agent is mid-turn (tui_gateway/server.py:_handle_busy_submit), so a
+ * message sent while the agent is typing or awaiting approval is never dropped —
+ * it runs as the next turn. These helpers keep that contract explicit and
+ * unit-testable without a Compose/Android harness.
+ */
+object ChatInputPolicy {
+    /**
+     * Whether the send affordance should be enabled.
+     *
+     * Unlike slash commands (which were always allowed mid-turn), regular
+     * prompts are now allowed too: the backend queues them. The only gates are
+     * a non-empty input (or a pending attachment) and an active connection.
+     */
+    fun canSend(
+        text: String,
+        pendingAttachments: List<Any>,
+        isConnected: Boolean,
+    ): Boolean = (text.isNotBlank() || pendingAttachments.isNotEmpty()) && isConnected
+
+    /**
+     * Whether the input placeholder should read "queued" rather than the plain
+     * "waiting" hint. Shown only while the agent is typing AND the user has
+     * already typed something — i.e. a press of send would enqueue a turn.
+     */
+    fun showQueuePlaceholder(
+        text: String,
+        isAgentTyping: Boolean,
+    ): Boolean = isAgentTyping && text.isNotBlank()
+}

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatScreen.kt

@@ -700,11 +700,12 @@ private fun ChatInputBar(
     onFileTap: () -> Unit = {},
     onRemoveAttachment: (Int) -> Unit = {},
 ) {
-    // Allow sending slash commands even while agent is typing
+    // Allow sending while the agent is mid-turn or awaiting approval: the
+    // gateway's prompt.submit busy-input policy queues it as the next turn
+    // (tui_gateway/server.py:_handle_busy_submit), so the message is never
+    // dropped. Slash commands were already allowed; regular prompts now are too.
     val isSlashCommand = inputText.startsWith("/")
-    val canSend =
-        (inputText.isNotBlank() || pendingAttachments.isNotEmpty()) &&
-            isConnected && (!isAgentTyping || isSlashCommand)
+    val canSend = ChatInputPolicy.canSend(inputText, pendingAttachments, isConnected)
 
     // Attachment menu state
     var showAttachmentMenu by remember { mutableStateOf(false) }
@@ -916,6 +917,8 @@ private fun ChatInputBar(
                             Text(
                                 if (!isConnected) {
                                     stringResource(R.string.chat_input_placeholder_not_connected)
+                                } else if (ChatInputPolicy.showQueuePlaceholder(inputText, isAgentTyping)) {
+                                    stringResource(R.string.chat_input_placeholder_queue)
                                 } else if (isAgentTyping) {
                                     stringResource(R.string.chat_input_placeholder_waiting)
                                 } else {