Route composite executor through llm-providers

[stackbilt-admin]

Summary

• routes composite Claude synthesis through buildLLMProviderFactory(env)
• routes composite Workers AI gather/tool rounds through provider toolCalls / toolResults
• forwards edgeEnv through composite dynamic-tool execution
• removes raw Anthropic fetch and direct env.ai.run() inference from composite.ts
• updates composite tests to mock provider gather/synthesis calls

Refs #24. After this, the only remaining raw provider endpoint in aegis-oss is the Groq logprobs classifier path, which needs provider-package support for logprob options before it can move cleanly.

Validation

• npm run typecheck
• npx vitest run tests/composite.test.ts tests/executors.test.ts
• npm test
• npx charter validate
• git -c core.whitespace=blank-at-eol,blank-at-eof,space-before-tab,cr-at-eol diff --check
• npm pack --dry-run