feat: defer broken manifest parse errors to preflight

Broken bottle/agent files no longer block the agent selector or prevent
unrelated agents from loading. Per-file parse errors are collected in
`Manifest.broken_agents`; the CLI selector includes them via
`all_agent_names`, and the error surfaces only when the specific agent
is selected and launch is attempted (in `require_agent`/`bottle_for`).

Closes #236

bot_bottle/cli/start.py

@@ -67,7 +67,7 @@ def cmd_start(argv: list[str]) -> int:
     agent_name: str | None = args.name
     if agent_name is None:
         agent_name = tui.filter_select(
-            sorted(manifest.agents.keys()),
+            manifest.all_agent_names,
             title="Select agent",
         )
         if agent_name is None: