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/destinations.ts

@@ -73,6 +73,22 @@ export function buildSystemPromptAddendum(): string {
     ].join('\n');
   }
 
+  // Single-destination shortcut: the agent just writes its response normally.
+  // No wrapping needed. This preserves the simple case (one user, one channel).
+  if (cache.length === 1) {
+    const d = cache[0];
+    const label = d.displayName && d.displayName !== d.name ? ` (${d.displayName})` : '';
+    return [
+      '## Sending messages',
+      '',
+      `Your messages are delivered to \`${d.name}\`${label}. Just write your response directly — no special wrapping needed.`,
+      '',
+      'To mark something as scratchpad (logged but not sent), wrap it in `<internal>...</internal>`.',
+      '',
+      'To send a message mid-response (e.g., an acknowledgment before a long task), call the `send_message` MCP tool.',
+    ].join('\n');
+  }
+
   const lines = ['## Sending messages', '', 'You can send messages to the following destinations:', ''];
   for (const d of cache) {
     const label = d.displayName && d.displayName !== d.name ? ` (${d.displayName})` : '';