fix: validate config check

[srobinson]

Summary

• centralize config validation rules and reuse them for runtime loading plus config check diagnostics
• make config check report malformed TOML, invalid values, effective coerced values, aiSummarization, and per-field validity
• emit all concrete schema fields in generated TOML, including embeddings retry/HNSW settings and paths.cacheDir

Test plan

• npx --yes pnpm@10.28.0 check
• npx --yes pnpm@10.28.0 build
• npx --yes pnpm@10.28.0 test
• manual invalid provider check returned exit code 1 with effective provider openai and one warning