fix(claude): remove global header cache, gate anthropic-beta by model

The global claudeHeaderCache singleton overlaid the last-seen Claude Code
client's identity headers onto every subsequent request, leaking one
client's headers (anthropic-beta, user-agent, x-stainless-*, etc.) onto
another client/account sharing the same server. Removed the singleton and
the claudeOverlay hook entirely, falling back to static per-provider
headers. anthropic-beta is now computed per-request from the requested
model, gating heavy-agent flags (advanced-tool-use, effort) to
opus/sonnet only.

open-sse/providers/registry/claude.js

@@ -49,9 +49,6 @@ export default {
         header: "Authorization",
         scheme: "bearer",
       },
-      hooks: [
-        "claudeOverlay",
-      ],
     },
     usage: {
       oauthUrl: "https://api.anthropic.com/api/oauth/usage",