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

tests/unit/test_cli_start_selector.py

@@ -20,6 +20,7 @@ from bot_bottle.backend import ActiveAgent
 def _make_manifest(agent_names: list[str]):
     manifest = MagicMock()
     manifest.agents = {name: MagicMock() for name in agent_names}
+    manifest.all_agent_names = sorted(agent_names)
     return manifest