fix(ai-client): clean up regressions from OpenAI SDK migration and canvas frame fix (#74)

Three follow-ups to ef3b579 (OpenAI SDK migration) and ebe39ef (canvas frame):

- .env.example / config.ts / AGENTS.md: anthropic & google native protocols
  were removed with the Vercel AI SDK, but .env.example and AGENTS.md still
  advertised them. Rewrite the docs to point Claude/Gemini at their
  OpenAI-compatible endpoints (api.anthropic.com/v1,
  generativelanguage.googleapis.com/v1beta/openai), drop the dead Gemini
  "Nano Banana" image example, sync AGENTS.md (text/vision protocol list,
  image protocol list, the "OpenAI/Gemini via AI SDK" reference note), and
  append a short hint in readProvider() error message guiding
  anthropic/google users to openai_compatible instead of a bare rejection.

- chat.ts: drop the unsafe `as { prompt_tokens_details?: ... }` cast; read
  cached_tokens straight off the SDK's CompletionUsage type. Add a comment
  noting the OpenAI usage object reports cache reads only (no cache-write
  count), so the create cost the old AI SDK path logged is unrecoverable.

- PlayCanvas.tsx: revert <img key={imageUrl}> to key={imageUrl.slice(-48)}.
  The gpt-image/mock paths emit multi-MB data URIs; using the full string as
  React's reconciliation key adds avoidable diff overhead during the frequent
  re-renders. Matches the existing <audio> element's key convention.

Validation: pnpm typecheck passes. (pnpm lint fails on a pre-existing Next 16
`next lint` CLI issue, identical on staging — unrelated to this change.)

lib/config.ts

@@ -40,8 +40,14 @@ function readProvider(name: string): ProviderProtocol | undefined {
   if ((VALID_PROTOCOLS as readonly string[]).includes(v)) {
     return v as ProviderProtocol;
   }
+  // anthropic/google were removed with the Vercel AI SDK — nudge users who
+  // still set them toward the OpenAI-compatible endpoints (see .env.example).
+  const hint =
+    v === "anthropic" || v === "google"
+      ? ` — use openai_compatible with their OpenAI-compatible endpoint instead`
+      : "";
   throw new Error(
-    `Invalid ${name}: "${v}". Must be one of: ${VALID_PROTOCOLS.join(", ")}`,
+    `Invalid ${name}: "${v}". Must be one of: ${VALID_PROTOCOLS.join(", ")}${hint}`,
   );
 }