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/ai-client/chat.ts

@@ -7,6 +7,12 @@ export type ChatMessage = {
   content: string;
 };
 
+// Cache observability for the prompt-prefix caching that the Writer stable
+// prefix relies on. The OpenAI usage object reports only cached READS
+// (prompt_tokens_details.cached_tokens) and has no field for cache WRITES
+// (tokens written to the cache on a cold pass), so unlike the old AI SDK
+// path we can show the hit rate but not the create cost. cached_tokens lives
+// directly on the SDK's CompletionUsage type — no cast needed.
 function summarizeSdkUsage(
   tag: string,
   usage: OpenAI.Completions.CompletionUsage | undefined,
@@ -14,8 +20,7 @@ function summarizeSdkUsage(
   if (!usage) return `[cache] ${tag} no-usage`;
   const input = usage.prompt_tokens ?? 0;
   const output = usage.completion_tokens ?? 0;
-  const details = (usage as { prompt_tokens_details?: { cached_tokens?: number } }).prompt_tokens_details;
-  const cached = details?.cached_tokens;
+  const cached = usage.prompt_tokens_details?.cached_tokens;
   if (typeof cached === "number") {
     const rate = input > 0 ? ((cached / input) * 100).toFixed(1) : "n/a";
     return `[cache] ${tag} hit=${cached} input=${input} rate=${rate}% completion=${output}`;