[FEATURE]: Set universal AI_AGENT marker for subprocesses #40441

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Describe the enhancement you want to request

Set AI_AGENT=opencode in the environment inherited by subprocesses and tools started during an OpenCode session. If a caller already provides a non-empty AI_AGENT, preserve it so wrappers can identify themselves.

AI_AGENT is a universal marker used across coding agents. It gives downstream CLIs one stable signal for agent-friendly behavior—such as disabling prompts and pagers or choosing machine-readable output—without maintaining product-specific heuristics. OpenCode-specific environment variables can continue serving their existing purposes; this marker complements rather than replaces them.

The value should be advisory only: not telemetry, authentication, authorization, sandboxing, or a trust boundary. The proposed slug follows the standard's lowercase hyphen naming: opencode.

Standard: https://github.com/vercel/vercel/tree/main/packages/detect-agent#the-ai_agent-standard