docs: clarify custom Ollama embedding setup

[Helweg]

Summary

Clarify how to use custom Ollama embedding models through the README without changing provider behavior.

Changes

• document that the built-in ollama provider uses Ollama's native /api/embeddings endpoint for the default nomic-embed-text flow
• add README guidance for using other Ollama embedding models through the custom provider with http://127.0.0.1:11434/v1
• include a concrete config example and notes about the /v1 suffix, placeholder API keys, and matching model dimensions

Testing

How were these changes tested?

• Unit tests added/updated
• Manual testing performed
• Build passes (npm run build)
• Typecheck passes (npm run typecheck)
• Tests pass (npm run test:run)
• Lint passes (npm run lint)

Release Labels

• Added at least one release category label (feature, bug, performance, documentation, dependencies, refactor, test, chore, or skip-changelog)
• Added at most one semver label (semver:major, semver:minor, semver:patch) when needed

Related Issues

Fixes #49