feat: single-destination shortcut — no wrapping needed when there's only one

When an agent has exactly one configured destination, wrapping output in
<message to="..."> blocks is unnecessary. Plain text goes to the sole
destination automatically. This preserves the simple "just reply" flow
for the common case of one user on one channel.

Applies in three places:

- System prompt addendum: single-destination case gets a simplified
  explanation ("your messages are delivered to X, just write directly").
  Multi-destination case keeps the <message to="..."> syntax docs.

- Main output parser: if zero <message> blocks are found and there is
  exactly one destination, the entire cleaned text (with <internal>
  stripped) is sent to that destination.

- send_message / send_file MCP tools: `to` parameter is now optional.
  With one destination, omitted defaults to it. With multiple, omitting
  returns an error listing the options.

Multi-destination behavior is unchanged — explicit <message to="..."> is
still required, and untagged text is still scratchpad.

groups/global/CLAUDE.md updated to describe both cases.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/agent-runner/src/poll-loop.ts

@@ -1,4 +1,4 @@
-import { findByName } from './destinations.js';
+import { findByName, getAllDestinations, type DestinationEntry } from './destinations.js';
 import { getPendingMessages, markProcessing, markCompleted, type MessageInRow } from './db/messages-in.js';
 import { writeMessageOut } from './db/messages-out.js';
 import { touchHeartbeat, clearStaleProcessingAcks } from './db/connection.js';
@@ -296,11 +296,14 @@ function handleEvent(event: ProviderEvent, _routing: RoutingContext): void {
 /**
  * Parse the agent's final text for <message to="name">...</message> blocks
  * and dispatch each one to its resolved destination. Text outside of blocks
- * (including <internal>...</internal>) is scratchpad — logged but not sent.
+ * (including <internal>...</internal>) is normally scratchpad — logged but
+ * not sent.
  *
- * If the agent emits zero <message> blocks AND non-empty text, log a warning:
- * the agent produced output with no recipient. That's usually a bug in the
- * agent — the system prompt tells it to wrap user-visible text in blocks.
+ * Single-destination shortcut: if the agent has exactly one configured
+ * destination AND the output contains zero <message> blocks, the entire
+ * cleaned text (with <internal> tags stripped) is sent to that destination.
+ * This preserves the simple case of one user on one channel — the agent
+ * doesn't need to know about wrapping syntax at all.
  */
 function dispatchResultText(text: string, routing: RoutingContext): void {
   const MESSAGE_RE = /<message\s+to="([^"]+)"\s*>([\s\S]*?)<\/message>/g;
@@ -324,18 +327,7 @@ function dispatchResultText(text: string, routing: RoutingContext): void {
       scratchpadParts.push(`[dropped: unknown destination "${toName}"] ${body}`);
       continue;
     }
-
-    const platformId = dest.type === 'channel' ? dest.platformId! : dest.agentGroupId!;
-    const channelType = dest.type === 'channel' ? dest.channelType! : 'agent';
-    writeMessageOut({
-      id: generateId(),
-      in_reply_to: routing.inReplyTo,
-      kind: 'chat',
-      platform_id: platformId,
-      channel_type: channelType,
-      thread_id: null,
-      content: JSON.stringify({ text: body }),
-    });
+    sendToDestination(dest, body, routing);
     sent++;
   }
   if (lastIndex < text.length) {
@@ -346,6 +338,17 @@ function dispatchResultText(text: string, routing: RoutingContext): void {
     .join('')
     .replace(/<internal>[\s\S]*?<\/internal>/g, '')
     .trim();
+
+  // Single-destination shortcut: the agent wrote plain text and has exactly
+  // one destination. Send the entire cleaned text to it.
+  if (sent === 0 && scratchpad) {
+    const all = getAllDestinations();
+    if (all.length === 1) {
+      sendToDestination(all[0], scratchpad, routing);
+      return;
+    }
+  }
+
   if (scratchpad) {
     log(`[scratchpad] ${scratchpad.slice(0, 500)}${scratchpad.length > 500 ? '…' : ''}`);
   }
@@ -355,6 +358,20 @@ function dispatchResultText(text: string, routing: RoutingContext): void {
   }
 }
 
+function sendToDestination(dest: DestinationEntry, body: string, routing: RoutingContext): void {
+  const platformId = dest.type === 'channel' ? dest.platformId! : dest.agentGroupId!;
+  const channelType = dest.type === 'channel' ? dest.channelType! : 'agent';
+  writeMessageOut({
+    id: generateId(),
+    in_reply_to: routing.inReplyTo,
+    kind: 'chat',
+    platform_id: platformId,
+    channel_type: channelType,
+    thread_id: null,
+    content: JSON.stringify({ text: body }),
+  });
+}
+
 function sleep(ms: number): Promise<void> {
   return new Promise((resolve) => setTimeout(resolve, ms));
 }