AD-802: Implement Patch Proposal Model

[alph-cmky]

Source: docs/development/task-backlog.md

Milestone: Phase 8: Workflow Canvas and Patch Approval

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AD-802: Implement Patch Proposal Model

Goal: Represent proposed agent/workflow changes without directly applying them.

Files:

• Create apps/server/src/patches/patch-schema.ts
• Create apps/server/src/patches/patch-service.ts
• Create apps/server/src/patches/patch-service.test.ts

Tasks:

• Define PatchProposal with target type, target ID, base version, JSON patch, validation result, approval state, and diff preview.
• Validate workflow patches through workflow-core.
• Require approval before apply.
• Commit with message feat: add patch proposal service.

Acceptance Criteria:

• A valid patch can be proposed and previewed without changing the target workflow.