Rename worker package cross_py_builder -> agent

The nested cross_py_builder package only contains the worker agent. Rename
it to agent/ to match its role and distinguish it from the ctrl package and
the PyPI distribution name. Internal imports are relative, so build_agent.py
and zeroconf_server.py are unaffected except for the cosmetic APP_NAME banner.

DESIGN.md

@@ -146,7 +146,7 @@ user resubmit to an eligible worker.
 ```
 setup.py                      (add ctrl package + entry point `cross-py-controller`)
 requirements.txt              (add waitress; keep Flask/requests)
-cross_py_builder/             (agent unchanged except new /progress endpoint)
+agent/                    (worker agent: build_agent.py, zeroconf_server.py)
 ctrl/
   __init__.py
   app.py           (Flask factory, routes, waitress runner)

agent/build_agent.py

@@ -19,7 +19,7 @@ import uuid
 import platform
 from .zeroconf_server import ZeroconfServer
 
-APP_NAME = "cross_py_builder"
+APP_NAME = "agent"
 build_agent_version = "0.1.34"
 app_port = 9001
 

setup.py

@@ -7,7 +7,7 @@ setup(
     install_requires=open("requirements.txt").read().splitlines(),
     entry_points={
         "console_scripts": [
-            "cross-py-agent=cross_py_builder.build_agent:main",
+            "cross-py-agent=agent.build_agent:main",
             "cross-py-controller=ctrl.app:main",
         ],
     },