docs: XERJ + Google AI — EmbeddingGemma (verified), Gemini API, ADK - #7

Worked, verified examples of using XERJ with Google's AI stack, in
docs/examples/google-ai/. XERJ speaks the OpenAI-compatible /v1/embeddings
contract and an ES REST/MCP surface, which is what Google's embedding API and
agent framework plug into.

Contents, each labelled by how it was checked:

1. EmbeddingGemma (google/embeddinggemma-300m, Google's open model) —
   embeddinggemma.toml points XERJ's proxy at Ollama's OpenAI endpoint for
   fully local, key-free embeddings. VERIFIED END-TO-END: four docs indexed
   through EmbeddingGemma, then three semantic queries with NO shared words
   with their answers all matched correctly (0.878 / 0.876 / 0.902) — real
   neural understanding, not lexical overlap.

2. Gemini Embeddings API (gemini-embedding-001) — gemini-embeddings.toml uses
   the endpoint from Google's OpenAI-compatibility docs
   (generativelanguage.googleapis.com/v1beta/openai/embeddings). Config matches
   the documented contract; the contract itself is proven by the offline mock
   test and the EmbeddingGemma run (identical OpenAI shape). Running the managed
   API additionally needs a GEMINI_API_KEY.

3. ADK agent (adk_xerj_agent.py) — a Google ADK Agent (gemini-2.5-flash) whose
   search_documents tool queries XERJ and returns cited snippets. VERIFIED: the
   agent constructs and the tool returns real cited results from the
   EmbeddingGemma-backed index. The full Gemini reasoning loop needs a
   GOOGLE_API_KEY; the XERJ tool integration does not.

4. run-offline-test.sh + mock_embed_server.py — a key-free, dependency-free
   proof of the whole path: mock /v1/embeddings server, XERJ in proxy mode,
   external API called at ingest AND query time.

Honesty notes kept in the README: EmbeddingGemma and the ADK tool are verified
here; the managed Gemini API config is verified structurally (matches Google's
documented contract, same shape as the runs that do work) and needs a key to
execute. Endpoints/model names change — the README links each Google source.

Docs/example only; no engine code changed.