feat: add canonical external agent runtime - #4

Summary

• add a runtime-neutral session API and authenticated HTTP adapter for external agent frameworks
• preserve canonical EcommerceToolManager execution, environment dynamics, logging, termination, and scoring
• share benchmark job construction with the built-in agent to prevent configuration drift
• document HTTP and in-process integration plus deployment isolation

Verification

• python3 -m unittest tests.test_external_runtime -v (7 passed)
• python3 -m compileall agent external_agent_server.py tests
• git diff --check
• live HTTP session/action/finish/result smoke test