Add Atlas Cloud as a supported LLM provider

[anhnh2002]

Summary

Adds Atlas Cloud as a first-class, branded LLM provider
(--provider atlas-cloud). Atlas Cloud is a full-modal AI inference platform that exposes
LLM, image, and video models (300+) behind a single OpenAI-compatible API, so this
integration reuses CodeWiki's existing OpenAI-compatible code path — no new backend and
no new dependency.

Selecting atlas-cloud adds two conveniences over the generic openai-compatible flow:

• the base URL is auto-set to https://api.atlascloud.ai/v1, and
• the API key is read from ATLASCLOUD_API_KEY when --api-key is omitted.

Changes

• codewiki/cli/commands/config.py — add atlas-cloud to the --provider choices and
  help text; add a convenience block that fills the base URL and reads ATLASCLOUD_API_KEY
  when those flags are omitted (values still pass through the existing validate_url /
  validate_api_key helpers); add a docstring example; import os.
• codewiki/src/config.py — add the ATLAS_CLOUD_BASE_URL constant and list
  atlas-cloud in the provider docstrings.
• codewiki/cli/models/config.py — list atlas-cloud in the Configuration docstring.
• README.md — list Atlas Cloud among the supported providers, add a config example, and
  a short "About Atlas Cloud" blurb with links (equal footing with the other providers).