feat: add Astraflow provider support

[ucloudnb666]

Summary

• Adds hooks/astraflow/README.md documenting Astraflow (by UCloud), an OpenAI-compatible AI model aggregation platform supporting 200+ models, with both global and China regional endpoints
• Adds an OpenAI-Compatible API Providers section to hooks/README.md listing Astraflow with both endpoints
• Integration requires zero new Rust code: users set OPENAI_API_KEY + OPENAI_BASE_URL to Astraflow credentials and run rtk init --opencode or rtk init --agent hermes as usual, achieving 60–90% token savings (= direct Astraflow API cost reduction)

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: documentation-only change; verified quick-start instructions against Astraflow's OpenAI-compatible endpoints (https://api-us-ca.umodelverse.ai/v1 global, https://api.modelverse.cn/v1 China) — no new code paths to test

Important: All PRs must target the develop branch (not master).
See CONTRIBUTING.md for details.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}