feat: Google (Gemini) provider on the official Gen AI SDK

Phase 4: provider/google on google.golang.org/genai v1.59.0 — lazy cached
client, FunctionResponse tool loop, raw-JSON-schema tools and structured
output, ThinkingLevel reasoning mapping, iter.Pull2 streaming, hermetic
httptest suite via HTTPOptions.BaseURL. Registry wires google + gemini
schemes to the real client; stub machinery deleted (all built-ins real).
ADR-0011; README matrix + CLAUDE.md synced.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -102,7 +102,7 @@ Chains are health-tracked per target:
 |----------|-----------|-------------|------------------|
 | OpenAI (+compatible) | `openai` | `OPENAI_API_KEY` | https://api.openai.com/v1 |
 | Anthropic (+compatible) | `anthropic` | `ANTHROPIC_API_KEY` | https://api.anthropic.com |
-| Google (Gemini) | `google` | `GOOGLE_API_KEY` / `GEMINI_API_KEY` | Gen AI API *(pending)* |
+| Google (Gemini) | `google` | `GOOGLE_API_KEY` / `GEMINI_API_KEY` | Gemini API (official SDK) |
 | Ollama Cloud | `ollama-cloud` | `OLLAMA_API_KEY` | https://ollama.com |
 | Ollama (local) | `ollama` | — | `OLLAMA_HOST` or http://localhost:11434 |
 | foreman | `foreman` | — (token via DSN) | requires an LLM_* DSN or `ollama.Foreman(url, token)` |
@@ -214,7 +214,7 @@ skills = reusable instruction+tool bundles attachable to any agent.
 |----------------------|:---:|:---:|:---:|:---:|:---:|:---:|:---:|
 | OpenAI (+compatible) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
 | Anthropic (+compat) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
-| Google (Gemini) | ✅ | pending | pending | pending | pending | pending | ✅ |
+| Google (Gemini) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
 | Ollama Cloud | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
 | Ollama (local) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
 | foreman | ✅ | ✅ | ✅¹ | ✅ | ✅ | ✅ | ✅ |