feat(claude): add extra_args config for upstream CLI flags (#407)

[Nathan Schram (nathanschram)]

Summary

Adds [claude] extra_args: list[str] to untether.toml so users can pass upstream claude CLI flags (like --chrome) without forking Untether. Primary motivator: Claude Code 2.1.x gates the mcp__claude-in-chrome__* tool namespace behind --chrome / CLAUDE_CODE_ENABLE_CFC=1, so Untether-spawned sessions never saw those tools. Setting extra_args = ["--chrome"] fixes it end-to-end.

Mirrors codex.extra_args and pi.extra_args.

• Untether-managed flags (-p, --print, --output-format, --input-format, --resume/-r, --continue/-c, --permission-mode, --permission-prompt-tool) are rejected at config-load with a ConfigError so duplicate-argv surprises fail fast.
• User args land on argv after the managed stream-json prelude and before resume/model/effort/allowed-tools/permission flags — the trailing -p <prompt> (or stdin prompt under permission-mode) position is never displaced.

Closes #407.

Test plan

• uv run pytest tests/test_build_args.py tests/test_claude_runner.py tests/test_claude_control.py (216 passed)
• uv run ruff check src/ + uv run ruff format --check src/ tests/
• python3 scripts/validate_release.py
• Empirical verification (Nathan): run claude --chrome -p "list your tools" --output-format stream-json --input-format stream-json --verbose < /dev/null on Mac to confirm --chrome works in stream-json mode before staging. Binary analysis found no stream-json-specific gate on the --chrome code path, but SSH to the Mac can't run it because claude.ai OAuth lives in the login Keychain.
• After merge + dev install: extra_args = ["--chrome"] in ~/.untether-dev/untether.toml, restart untether-dev, prompt via @untether_dev_bot, confirm mcp__claude-in-chrome__* appears in the tool catalogue.

🤖 Generated with Claude Code

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