[codex] reset clean baseline prompt surface

[tryskian]

Summary

• reset the runtime prompt surface so config.py only carries structural constants, settings, and validation
• move generation to one minimal agent routing prompt and remove the research beta label from the CLI banner
• document the clean-baseline reset boundary while preserving Beta 6.0 as a diagnostic snapshot

Why

The first Beta 6.0 pulse remains useful evidence, but prior prompt scaffolding made it unsafe to treat as clean baseline proof. This keeps the eval machinery and tracked beta history while stripping the live prompt/config surface back to a proper-config candidate.

Validation

• make test TEST_ARGS="tests/test_agent.py tests/test_config.py tests/test_main.py"
• make format-check
• make lint-docs
• git diff --check
• make check
• make package-check
• make package-install-check
• make security-checks