hotfix: wire the xai provider into install-opencode.sh

PR #114 pointed @lead/@ops at xai/grok-4.5 but never gave opencode a
provider config for it — xai is not a built-in like anthropic was, it
needs the same explicit OpenAI-compatible block as ollama/ollama-cloud
(confirmed by the prior, never-merged investigation in PR #110). Without
this, any @lead/@ops run fails immediately on an unresolvable provider.

Adds the xai provider block (baseURL https://api.x.ai/v1, models derived
from agents.json) and passes XAI_API_KEY into the Install-opencode step
(it was already added to the Run-agent step in #114).

.gitea/workflows/agent.yml

@@ -149,6 +149,7 @@ jobs:
           SCRIPTS: ${{ runner.temp }}/agents-scripts
           OLLAMA_URL: ${{ secrets.OLLAMA_URL }}
           OLLAMA_CLOUD_API_KEY: ${{ secrets.OLLAMA_CLOUD_API_KEY }}
+          XAI_API_KEY: ${{ secrets.XAI_API_KEY }}
           NAME: ${{ steps.prep.outputs.name }}
           SKILLS: ${{ steps.prep.outputs.skills }}   # JSON array of skills this agent may load
         run: bash "$SCRIPTS/install-opencode.sh"