Remove final raw Groq inference path

[stackbilt-admin]

Summary

• removes the final raw Groq chat-completions endpoint from groq.ts
• keeps askGroqWithLogprobs API-compatible for router callers by using provider-backed JSON classification
• maps tokenConfidence to self-reported confidence until llm-providers grows explicit logprob request support
• updates Groq tests to cover provider-backed confidence classification

Fixes #24.

Validation

• npm run typecheck
• npx vitest run tests/groq.test.ts tests/router.test.ts tests/dispatch.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