Add an xai provider (OpenAI-compatible, https://api.x.ai/v1) to
install-opencode.sh, with its models map derived from agents.json
exactly like the ollama providers. Pass XAI_API_KEY into both the
Install-opencode and Run-agent steps of agent.yml. Switch @lead's
model from anthropic/claude-opus-4-8 to xai/grok-4.5 (vision-capable,
so @lead keeps image reading). All other agents unchanged;
ANTHROPIC_API_KEY is retained for @ops and not removed.
senior