Support temporary global custom request headers for Claude Code

[andy-xu-db]

🥞 Stacked PR

Use this link to review incremental changes.

• stack/andy/custom-request-headers [Files changed]
  • stack/andy/custom-request-headers-claude [Files changed] ← this PR

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Summary

• add repeatable ug claude --header "Name: value" support
• reuse the existing private and OS-managed settings reconciliation required by Claude precedence
• reject protected headers and collisions with existing user or admin headers
• record exact temporary values in the existing global state and remove only those values on the next header-free launch or configuration
• document that the setting applies globally to active Claude sessions until it is cleared

Stack

Stacked on #610, which contains the shared parser and launch plumbing. Merge #610 first.

Testing

• uv run pytest -q tests/test_state.py tests/test_agent_claude.py tests/test_cli.py tests/test_agent_codex.py tests/test_codex_smart_routing_v2.py (734 passed)
• full suite passes with tests/test_e2e_user_agent.py excluded; its installed Claude 2.1.273 rejects the synthetic test model before making a request
• uv run ruff check src tests
• uv run ruff format --check src tests
• git diff --check