feat(opencode): add Terra execution subagent

[xnoto]

Summary

Adds terra, a generic OpenCode execution subagent for bounded coding, debugging, and repository tasks. Its routing description explicitly covers the case where the assigning primary agent's execution budget is nearing completion. Terra uses openai/gpt-5.6-terra with the default variant; the assigning primary retains task interpretation, safety, delivery decisions, and final synthesis.

Fixes #
N/A — owner-approved OpenCode agent capability.

Type of change

• Bug fix
• Feature / enhancement
• Documentation
• Infrastructure (OpenTofu root or module)
• GitOps desired state (manifests, kustomize, charts, SOPS/KSOPS secrets)
• Container image
• CI / reusable workflow
• Refactor / cleanup
• Breaking change

Validation

• Required pull-request checks pass — repository hygiene and Helm validation, changed-chart detection, and opencode-server package build succeeded.
• Generated or centrally distributed files were regenerated by their owning automation, not hand-edited — files/agents/*.md is packaged and mounted by the existing Helm .Files.Glob template; no generated source was edited.

The chart test target asserts the exact Terra definition, chart version, unchanged default-agent selection, rendered ConfigMap and mount entries, and packaged agent-file path. PR checks validate authored chart content only; runtime agent selection and model availability require later rollout verification.

Impact and rollout

Producer changed: opencode-server chart version 0.2.0.

After the approved merge and successful main package job, charts publishes immutable oci://ghcr.io/makeitworkcloud/charts/opencode-server:0.2.0. Existing automation then creates or updates the separate makeitworkcloud/kustomize-cluster OpenCode version-pin PR. Its required checks and auto-merge govern desired-state selection. That PR does not itself prove Argo CD reconciliation, workload health, or Terra behavior.

After the GitOps pin merges, validate the root and child Application revisions and health, the Deployment rollout, and a fresh OpenCode session that can select Terra for a bounded task. Rollback is a manual GitOps pin to an earlier immutable chart version.

Safety and secrets

• Contains no plaintext secrets, decrypted SOPS values, state files, kubeconfigs, tokens, or private endpoints
• No local OpenTofu init/plan/apply/destroy/import/state operations were run or claimed — plans come from pull-request checks
• Breaking or irreversible effects are described above with rollback notes

AI-assisted change; reviewer scrutiny requested.

[github-actions]

Chart CI passed

Repository hygiene and Helm validation passed. View the workflow run.