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>

docs/adr/0011-google-provider.md

@@ -0,0 +1,46 @@
+# ADR-0011: Google provider on the official Gen AI SDK
+
+**Status:** Accepted — 2026-06-10
+
+## Context
+
+ADR-0007 approves exactly one third-party dependency: Google's surface
+(auth modes, API versions, endpoint shapes) moves too much to hand-roll
+profitably. The deprecated `github.com/google/generative-ai-go` is not an
+option; `google.golang.org/genai` is the current official SDK.
+
+## Decision
+
+- Build `provider/google` on `google.golang.org/genai` **v1.59.0**,
+  `BackendGeminiAPI`, API key from `GOOGLE_API_KEY` then `GEMINI_API_KEY`
+  (the SDK's own precedence).
+- The SDK client is created **lazily on first request** and cached;
+  construction of the Provider never fails (per ADR-0005). A missing key
+  is a synthetic 401 `APIError`, so chains fail over past it.
+- Mappings (recorded in the package doc): assistant role → `model`;
+  tool results → `FunctionResponse` parts in a user content
+  (`{"output": ...}` / `{"error": ...}` payloads); tool schemas via
+  `FunctionDeclaration.ParametersJsonSchema` (raw JSON schema — no lossy
+  conversion to genai.Schema); structured output via `ResponseJsonSchema`
+  + `application/json` MIME; `ToolChoice` → `FunctionCallingConfig` modes
+  (`required` → ANY, named → ANY + AllowedFunctionNames, `none` → tools
+  omitted); `ReasoningEffort` → `ThinkingConfig.ThinkingLevel`
+  (LOW/MEDIUM/HIGH); usage output = candidates + thoughts tokens; thought
+  parts are skipped in content.
+- Streaming adapts the SDK's `iter.Seq2` to majordomo's pull-based
+  `Stream` with `iter.Pull2`; `Close` releases the iterator via its stop
+  function. Function calls arrive whole per chunk (no partial-args
+  assembly needed).
+- Hermetic tests use the SDK's documented hooks:
+  `HTTPOptions.BaseURL` + `HTTPClient` pointed at `httptest` servers (the
+  same technique as the SDK's own test suite); streaming fixtures are SSE.
+- Errors: `genai.APIError` (value type; `Code` = HTTP status) maps to
+  `llm.APIError{Status: Code, Code: Status}` so the standard classifier
+  applies.
+
+## Consequences
+
+- `go.mod` gains genai and its transitive tree (auth, grpc, protobuf) —
+  the one sanctioned dependency cost.
+- Vertex AI backend is NOT wired (API-key Gemini only); adding it later is
+  an options-level change, not a redesign.