feat(agent): default to GLM-5.3 with Kimi override

[matteius]

Summary

• Use GLM-5.3 for labeled issue agents by default.
• Select Kimi K3 when agent-kimi is already on the issue before agent-candidate starts the session.
• Record the selected model in the Actions run summary and document the label order for maintainers.

Validation

• Both DigitalOcean manifests passed client-side doctl harness-runtime validate with placeholder credentials.
• The workflow's embedded Bash passed bash -n; the label expression was checked for both model paths.
• No paid agent session was started.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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