docs(providers): note that ANTHROPIC_API_KEY requires an API account, not a subscription

Claude Max and Pro subscription users authenticate via OAuth, not an API
key, causing a silent failure when creating the claude provider. Add a
Note callout in the provider type table and quickstart guide directing
subscription users to generate an API key from console.anthropic.com.

Closes #620

Author: mesutoezdil

docs/get-started/quickstart.mdx

@@ -60,6 +60,10 @@ The CLI prompts you to create a provider from local credentials.
 Type `yes` to continue.
 If `ANTHROPIC_API_KEY` is set in your environment, the CLI picks it up automatically.
 If not, you can configure it from inside the sandbox after it launches.
+
+<Note>
+`ANTHROPIC_API_KEY` is an API key from [console.anthropic.com](https://console.anthropic.com), not a Claude.ai subscription token. Claude Max and Pro subscription users must generate a separate API key.
+</Note>
 </Tab>
 
 <Tab title="OpenCode">

docs/sandboxes/manage-providers.mdx

@@ -246,6 +246,10 @@ The following provider types are supported.
 |---|---|---|
 | `anthropic` | `ANTHROPIC_API_KEY` | Anthropic API |
 | `claude` | `ANTHROPIC_API_KEY`, `CLAUDE_API_KEY` | Claude Code, Anthropic API |
+
+<Note>
+`ANTHROPIC_API_KEY` is an API key from [console.anthropic.com](https://console.anthropic.com), not a Claude.ai subscription token. Claude Max and Pro subscription users must generate a separate API key from the Anthropic Console.
+</Note>
 | `codex` | `OPENAI_API_KEY` | OpenAI Codex |
 | `copilot` | `COPILOT_GITHUB_TOKEN`, `GH_TOKEN`, `GITHUB_TOKEN` | GitHub Copilot CLI |
 | `generic` | User-defined | Any service with custom credentials |