feat: basic prepare deploy action

[Che-Zhu]

Summary

• replace the terminal Run App action with a basic Prepare Deploy action
• invoke /fulling-deploy in the selected sandbox directory via the existing background exec API
• update the confirmation dialog copy to describe this basic deploy-prep workflow

Notes

• this is the basic version only
• it starts the skill, but does not yet expose task progress, logs, or completion state in the UI

[github-actions]

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  Commit: 6b9e8a35656c5d50d1484868dc104c19e9fd9198
  Branch: codex/prepare-deploy

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