feat: LLMProviders.fromEnv(env) auto-discovery constructor

[stackbilt-admin]

Summary

Add a convenience factory method that auto-discovers available LLM providers from a Cloudflare Worker env object, eliminating manual config assembly.

Motivation

From the frontend team (Oracle project): they're manually checking for each key (env.ANTHROPIC_API_KEY, env.OPENAI_API_KEY, env.AI, etc.) and building the config object by hand. This is boilerplate that the package should handle.

Proposed API

// Auto-discover all available providers from worker env
const llm = LLMProviders.fromEnv(env);

// With overrides
const llm = LLMProviders.fromEnv(env, {
  defaultProvider: 'anthropic',
  costOptimization: true,
});

Detection logic

Scan env for known keys:

• env.ANTHROPIC_API_KEY → configure Anthropic provider
• env.OPENAI_API_KEY → configure OpenAI provider
• env.GROQ_API_KEY → configure Groq provider
• env.CEREBRAS_API_KEY → configure Cerebras provider
• env.AI (binding) → configure Cloudflare Workers AI provider

Acceptance criteria

• Static method LLMProviders.fromEnv(env, overrides?)
• Only configures providers whose keys/bindings are present
• Supports optional overrides for defaultProvider, costOptimization, etc.
• Unit tests covering partial env (e.g., only 2 of 5 providers available)
• Exported from package entry point

[stackbilt-admin]

Shipped in v1.0.0 (commit 743a1e0). LLMProviders.fromEnv() scans for ANTHROPIC_API_KEY, OPENAI_API_KEY, GROQ_API_KEY, CEREBRAS_API_KEY, and AI binding.