feat(agent): implement bounded ComfyUI workflow construction, DAG review, and validation gating

[BerryUIKI]

Summary

Addresses product gap in Agent workflow construction:

1. Bounded Supported Workflows: Defines 6 bounded ComfyUI workflow archetypes in \WorkflowCatalog\ (\comfy.txt2img.standard, \comfy.img2img.standard, \comfy.inpaint.standard, \comfy.upscale.esrgan, \comfy.img2video.svd, \comfy.txt2video.animatediff).
2. Reviewable Proposed Graph: Structured reviewable summary (\ReviewableWorkflowGraph\ and \WorkflowNodeStage) presenting execution stages, required nodes, and required models to the user.
3. Canvas Isolation: Keeps raw ComfyUI ports, handles, and internal tensor wirings off the primary creative canvas while providing transparent DAG stage breakdown in the Agent Proposal panel.
4. Validation and Recovery Guidance: Pre-execution dependency and DAG validation via \WorkflowValidator. Detects missing model checkpoints/modules and returns explicit recovery guidance (model paths or Cloud BYOK).
5. Execution Gate: Strictly enforces \�pproved=True\ and validity check before execution in \AgentService.execute_proposal.