fix(start): print error when no agents are defined

Previously start silently exited 0 when the manifest had no agents,
leaving the user with no feedback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/cli/start.py

@@ -64,6 +64,13 @@ def cmd_start(argv: list[str]) -> int:
 
     agent_name: str | None = args.name
     if agent_name is None:
+        if not manifest.all_agent_names:
+            print(
+                "bot-bottle: no agents defined. "
+                "Add an agent to ~/.bot-bottle/agents/ or ./bot-bottle/agents/ to get started.",
+                file=sys.stderr,
+            )
+            return 1
         agent_name = tui.filter_select(
             manifest.all_agent_names,
             title="Select agent",