feat(auto_name): propagate first chat name to parent session

When a chat is auto-named, also rename the parent session if it is
still on its default 'New session' label. UPDATE is gated by an
atomic WHERE clause so user renames and prior propagations are not
clobbered. Publishes session_renamed via broker.publishUser; useSidebar
already listens.

Closes the gap where sessions auto-created from the sidebar would
stay 'New session' forever.

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

apps/server/src/services/auto_name.ts

@@ -144,4 +144,23 @@ export async function maybeAutoNameChat(
     updated_at: updated[0]!.updated_at,
   });
   ctx.log.info({ chatId, name }, 'chat auto-named');
+
+  // Propagate to the parent session if it's still on its default name.
+  // The WHERE guard makes the check atomic — if the user has already
+  // renamed (or a prior chat already propagated), this UPDATE matches
+  // zero rows and we do nothing. First chat wins; manual renames win.
+  const renamedSession = await ctx.sql<{ id: string; name: string }[]>`
+    UPDATE sessions
+    SET name = ${name}
+    WHERE id = ${sessionId} AND name = 'New session'
+    RETURNING id, name
+  `;
+  if (renamedSession.length > 0) {
+    ctx.publishUser({
+      type: 'session_renamed',
+      session_id: sessionId,
+      name,
+    });
+    ctx.log.info({ sessionId, name }, 'session auto-named from chat');
+  }
 }