F-04 [High] CD still injects invalid gpt-5.4-mini (repo var; YAML default is now gpt-4o-mini)

## Summary\n\nThe "Build env var arguments" step falls back to `|| 'gpt-5.4-mini'` when the deployment name variable is unset. `gpt-5.4-mini` is **not a valid Azure OpenAI deployment name** — every deployment where this variable is not explicitly provided fails at AI client initialization at runtime.\n\n## Affected Files\n\n- `.github/workflows/deploy-containerapp-stage.yml` — "Build env var arguments" step\n\n## Impact\n\nSilent runtime failure on any staging deployment where the AI deployment name variable is not explicitly set.\n\n## Fix\n\n- Change default to `gpt-4o-mini` (matches `appsettings.json`), **or**\n- Mark the input variable `required: true` to fail fast at dispatch time.\n\n## References\n\n- Audit artifact: `spin-agent-full-audit.md` (project artifact 1e910fcc580a4cb0)\n- Auditor: Thor

Open .github/workflows/deploy-containerapp-stage.yml and find the “Build env var arguments” step, then compare its fallback with appsettings.json. Verify the chosen fix handles an unset deployment-name variable and that the workflow no longer passes the invalid gpt-5.4-mini value.

Written by the indexing model from the issue text.