The anonymous / free-chat agent is a distinct live agent (not part of the
single-agent stack and not shared infrastructure), so it gets its own top-level
package app/agents/anonymous_chat/ (sibling to multi_agent_chat). Moved
new_chat/anonymous_agent.py -> anonymous_chat/agent.py with a package __init__
re-exporting create_anonymous_chat_agent + build_anonymous_system_prompt.
Repointed its only new_chat import (the context shim) to app.agents.shared.context
and updated the single importer (anonymous_chat_routes). The test_import_all
guard auto-discovers the new package via pkgutil.walk_packages, so it is covered.
