build.yml: missing Dockerfile.copilot in CI matrix

[thepagent]

PR #265 added Dockerfile.copilot but did not update .github/workflows/build.yml to include it in the build matrix.

Currently the matrix only builds 4 variants:

• Dockerfile (default)
• Dockerfile.codex
• Dockerfile.claude
• Dockerfile.gemini

Need to add { suffix: "-copilot", dockerfile: "Dockerfile.copilot", artifact: "copilot" } to all three matrix blocks:

1. build-image
2. merge-manifests
3. promote-stable

Without this, ghcr.io/openabdev/openab-copilot is never published by CI.