Add Atlas Cloud provider profile

[binyangzhu000-sudo]

Summary

• add an Atlas Cloud OpenAI-compatible provider profile with its own atlascloud_api_key auth source
• support ATLASCLOUD_API_KEY / OPENHARNESS_ATLASCLOUD_API_KEY in auth resolution and provider status
• add Atlas Cloud to the setup provider picker and model suggestions
• update the existing provider compatibility docs table only

Validation

• PYTHONPATH=src uv run --python /Users/zby/.local/bin/python3.11 --with pytest --with pytest-asyncio --with pydantic --with typer --with rich --with httpx --with anthropic --with openai --with pyyaml --with questionary --with croniter --with watchfiles --with websockets --with mcp --with pyperclip --with slack-sdk --with python-telegram-bot --with discord.py --with lark-oapi python -m pytest tests/test_config/test_settings.py::TestAtlasCloudProvider tests/test_commands/test_cli.py::test_setup_flow_creates_atlascloud_profile_with_profile_scoped_key -q
• PYTHONPATH=src uv run --python /Users/zby/.local/bin/python3.11 --with ruff ruff check src/openharness/config/settings.py src/openharness/auth/manager.py src/openharness/cli.py src/openharness/ui/backend_host.py tests/test_config/test_settings.py tests/test_commands/test_cli.py
• git diff --check
• PYTHONPATH=src /Users/zby/.local/bin/python3.11 -m compileall -q src/openharness tests/test_config/test_settings.py tests/test_commands/test_cli.py

Notes

• No sponsor block, logo wall, credits, or partner copy was added.
• README changes are limited to the existing provider compatibility section.
• Atlas Cloud model IDs were checked against the live public model catalog before opening this PR.