Add Atlas Cloud LLM provider

[binyangzhu000-sudo]

Summary

• add an Atlas Cloud LLM provider using the existing OpenAI-compatible chat completions path
• include Atlas Cloud in the default provider chain after OpenAI and before Gemini/Anthropic when ATLASCLOUD_API_KEY is configured
• support explicit --llm-provider atlascloud with optional --llm-base-url and --llm-model overrides

Validation

• rustfmt --edition 2024 --check crates/webclaw-llm/src/providers/atlascloud.rs
• reviewed compare diff for provider wiring and CLI error text

No README changes.