fix: add t3code to .github/workflows/docker.yml build matrix #3418

Context

sh/docker/t3code.Dockerfile was added in PR #3409 fix (and now also filed as the fix for the Docker gap), but .github/workflows/docker.yml build matrix still doesn't include t3code.

Impact

Users running any of the following hit a runtime failure because ghcr.io/openrouterteam/spawn-t3code:latest doesn't exist:

• spawn t3code <cloud> --beta docker
• spawn t3code <cloud> --beta sandbox
• Any user in the fast_provision experiment group choosing t3code

All other 9 agents (claude, codex, cursor, openclaw, opencode, kilocode, hermes, junie, pi) are in the matrix.

Fix

Add t3code to the matrix.agent list in .github/workflows/docker.yml (agent file off-limits for automated agents — requires human review).

Filed by refactor/code-health — workflow changes require manual review per project rules.