sbx: add OpenRouter provider to agent authentication docs (#25276)

## Summary

Adds `sbx secret set -g openrouter` and `OPENROUTER_API_KEY` to the
Authentication sections of `docker-agent.md` and `opencode.md`, matching
the OpenRouter support added in docker/sandboxes#3244. Also corrects the
Google env var in `opencode.md` from `GOOGLE_API_KEY` to
`GOOGLE_GENERATIVE_AI_API_KEY`, which is the name OpenCode actually
uses.

Closes #25273

Generated by Claude Code

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dvdksn

content/manuals/ai/sandboxes/agents/docker-agent.md

@@ -32,14 +32,16 @@ $ sbx secret set -g google
 $ sbx secret set -g xai
 $ sbx secret set -g nebius
 $ sbx secret set -g mistral
+$ sbx secret set -g openrouter
 ```
 
 You only need to configure the providers you want to use. Docker Agent detects
 available credentials and routes requests to the appropriate provider.
 
 Alternatively, export the environment variables (`OPENAI_API_KEY`,
 `ANTHROPIC_API_KEY`, `GOOGLE_API_KEY`, `XAI_API_KEY`, `NEBIUS_API_KEY`,
-`MISTRAL_API_KEY`) in your shell before running the sandbox. See
+`MISTRAL_API_KEY`, `OPENROUTER_API_KEY`) in your shell before running the
+sandbox. See
 [Credentials](../security/credentials.md) for details on both methods.
 
 ## Configuration

content/manuals/ai/sandboxes/agents/opencode.md

@@ -42,13 +42,15 @@ $ sbx secret set -g google
 $ sbx secret set -g xai
 $ sbx secret set -g groq
 $ sbx secret set -g aws
+$ sbx secret set -g openrouter
 ```
 
 You only need to configure the providers you want to use. OpenCode detects
 available credentials and offers those providers in the TUI.
 
 You can also use environment variables (`OPENAI_API_KEY`, `ANTHROPIC_API_KEY`,
-`GOOGLE_API_KEY`, `XAI_API_KEY`, `GROQ_API_KEY`, `AWS_ACCESS_KEY_ID`). See
+`GOOGLE_GENERATIVE_AI_API_KEY`, `XAI_API_KEY`, `GROQ_API_KEY`,
+`AWS_ACCESS_KEY_ID`, `OPENROUTER_API_KEY`). See
 [Credentials](../security/credentials.md) for details on both methods.
 
 ## Configuration