fix: package Compute chart for CI image build

[Sau1707]

Summary

• Package the Compute Helm chart in the API image CI job.
• Pass the resulting archive to the Docker build via COMPUTE_ARCHIVE, matching the required Dockerfile input.

Verification

• helm package k8s/chart --version 0.0.0 --destination /tmp/opencode succeeded.
• git diff --check passed.
• Docker build could not be run locally because the Docker daemon is unavailable; the PR image job will validate it.