feat: add codex responses provider

[MarsDoge]

Summary

• add codex-responses provider using the Responses API with streaming SSE parsing
• document provider usage and expose it in CLI provider help/listing
• add static provider test covering payload, endpoint, headers, streaming parser, and provider discovery

Tests

• bash -n bin/llm-translate lib/stream.sh lib/providers/codex-responses.sh tests/codex-responses-static.sh
• shellcheck bin/llm-translate lib/stream.sh lib/providers/codex-responses.sh tests/codex-responses-static.sh
• ./tests/codex-responses-static.sh
• ./bin/llm-translate --list-providers | grep -qx codex-responses

Secret review

• Reviewed staged diff for token patterns; only placeholder/test values and environment variable references were present.