Support custom request headers for Codex and Claude

[andy-xu-db]

Summary

• add a repeatable --header 'Name: value' option to Codex and Claude launches
• validate header syntax, protect authentication and transport headers, and display names without values
• keep Codex headers launch-only and clean up Claude headers on the next header-free configuration

Usage

ug codex --header 'X-Development-Route: test-target'
ug claude --header 'X-Development-Route: test-target'

Testing

• uv run pytest -q tests/test_cli.py tests/test_agent_codex.py tests/test_agent_claude.py tests/test_codex_smart_routing_v2.py (622 passed)
• uv run ruff check src tests
• uv run ruff format --check src tests
• git diff --check