Clarify config and auto-capture docs

[x-Spartacus]

Closes #105

• Document first-use/source install prerequisites for embeddings and dependencies
• Clarify that the README config is a trimmed example and the full commented template is generated on first startup
• Document auto-capture provider requirements, opencodeProvider precedence, manual provider modes, proxy/tool-call limitations, and temperature troubleshooting

Tested:

• npx prettier --check README.md
• npx bun run typecheck