ci: register explicit OpenCode inference trial

[erikleblansch]

Scope

Register only the manual workflow file on the default branch so GitHub can dispatch it against the reviewed OpenCode feature branch. This does not merge the runtime migration or claim successful coding inference.

The file is byte-identical to feature/opencode-integration at e021e5ca52bf8bd23c857ba2917a2ddfe567eb15. Dispatch requires that feature commit as expected_code_sha; exact clean source checks precede ephemeral host changes. An unprepared main dispatch cannot pass the source stage.

Boundaries

• Manual-only, contents-read, no inference on PR/push.
• Fixed core/source/model provenance; existing guest task and memory limits unchanged.
• Scoped KVM/AppArmor setup and cleanup only on a disposable GitHub host.
• Closed provenance/result/cleanup artifacts only, no keys, VM images or raw runtime logs.

Verification

Independent source review, seven new and five existing offline Python contracts, shell syntax/ShellCheck, workflow parsing and diff checks pass on the feature implementation. Hosted admission and real Qwen code editing/testing remain unproved and are the purpose of the subsequent single trial.

GitHub requires the manual workflow on the default branch: https://docs.github.com/en/actions/how-tos/manage-workflow-runs/manually-run-a-workflow