agent: document Vercel, Mistral, llama.cpp providers and effort defaults

[arrufat]

Summary

Updates the lightpanda agent docs to match recent changes in the browser and zenai repos. The agent gained new LLM providers and effort behavior that the docs didn't reflect.

Providers

• Vercel AI Gateway — --provider vercel, env AI_GATEWAY_API_KEY
• Mistral — --provider mistral, env MISTRAL_API_KEY
• llama.cpp — --provider llama_cpp, local (no key), base http://localhost:8080/v1, probed alongside Ollama

Note the Vercel flag is vercel even though its env var is AI_GATEWAY_API_KEY.

Other changes

• API-key auto-detection order extended with AI_GATEWAY_API_KEY → MISTRAL_API_KEY
• Documented HF_BILL_TO for billing Hugging Face routed requests to an org
• Explained effort resolution order (flag > remembered > provider default > mode default); Mistral now defaults to none because its default model rejects reasoning effort

Files

• src/content/usage/agent.mdx — providers table, auto-detect order, local probing, HF billing, effort
• src/content/run-locally/commands/agent.mdx — --provider allowed values & env lists, --base-url, --effort defaults

Note for maintainers

browser/src/help.zon (the source of the --help text mirrored in commands/agent.mdx) is also stale — it still lists only anthropic, openai, gemini, huggingface, ollama and omits the new env vars. These docs now reflect reality; a follow-up in the browser repo should refresh help.zon so --help and the docs reconverge.

Verification

Verified every flag / env var / default against the source: zenai/src/provider.zig (openAiPreset, default_candidates, defaultEffort), browser/src/agent/settings.zig (api_keys_hint, resolveEffort), and browser/src/Config.zig (agent option set). Biome/typecheck can't run here (the package needs the parent monorepo workspace for @lightpanda/common); changes are pure markdown prose/tables.