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>

CLAUDE.md

@@ -35,7 +35,9 @@ majordomo        Registry, Parse, env-DSN loading, chain executor, re-exports
   provider/anthropic/ Messages API client (+ Anthropic-compat targets)
   provider/ollama/    one native /api/chat client serving the ollama,
                       ollama-cloud, and foreman built-ins via presets
-  provider/google/    Gemini on the official genai SDK         (Phase 4)
+  provider/google/    Gemini on google.golang.org/genai (the one approved
+                      dependency; lazy client, raw-JSON-schema tools,
+                      ThinkingLevel reasoning, iter.Pull2 streaming)
   agent/         Agent run loop                               (Phase 5)
   skill/         Skill interface + composition                (Phase 6)
   examples/      one runnable example per hard requirement    (Phase 7-8)