feat(agents): wire xAI provider, move @lead to xai/grok-4.5 (issue #12)

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.

.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"
@@ -212,6 +213,7 @@ jobs:
         env:
           SCRIPTS: ${{ runner.temp }}/agents-scripts
           ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
+          XAI_API_KEY: ${{ secrets.XAI_API_KEY }}
           # SELF_TOKEN = the RUNNING agent's OWN token (TOKEN_PM for @pm, TOKEN_OPS for @ops, …).
           # Only this agent's token is placed in its process env, so no agent can act as another.
           # Powers the gitea-api / gitea-admin skills — each agent calls Gitea as itself. Every