[0.2.0v][Feature] OpenAI provider support for embeddings and LLM

[Kavirubc]

Overview

Add OpenAI as an alternative AI provider alongside Gemini, so users who don't have a Gemini API key can use Simili-Bot with OpenAI instead.

Scope

• Auto-select provider based on available API keys (GEMINI_API_KEY preferred, OPENAI_API_KEY as fallback)
• Support OPENAI_API_KEY across all workflow examples and docs
• Update embedder and LLM client initialisation to be provider-agnostic
• Update .env.sample, simili.yaml examples, and setup guides to document both providers
• Clarify supported embedding models and their dimensions per provider

Acceptance Criteria

• At least one of GEMINI_API_KEY or OPENAI_API_KEY must be set (validated at startup)
• Gemini is preferred when both keys are present
• Default OpenAI models documented (text-embedding-3-small for embeddings)
• All existing tests pass with the new provider abstraction
• No breaking changes for existing Gemini-only users

Notes

Extracted from PR #40. Implementation reference: nick1udwig/simili-bot@index-and-query-prs.