docs: surface img-forge retention policy for image_generate

[stackbilt-admin]

Context

img-forge now codifies and enforces a customer-content retention policy in the product gateway:

• Generated output images and job metadata: 30 days from job completion
• Inpainting masks / transient edit inputs: 24 hours from job completion
• Manual deletion: DELETE /v2/jobs/:id removes terminal job metadata and deletes the output R2 object when no remaining job references it
• Public policy endpoint: GET https://imgforge.stackbilt.dev/v2/retention-policy

The MCP gateway proxies img-forge through image_generate, so MCP users need to see the same retention disclosure at the point of use.

Current gap

docs/user-guide.md and docs/api-reference.md describe image_generate, but they do not mention retention or manual deletion. The user-guide sample also shows a stale asset URL shape (/images/abc123.png) instead of the current /v2/assets/<tenant>/<hash>.png shape.

Acceptance criteria

• Update MCP gateway docs for image_generate to state the img-forge retention policy:
  • outputs/job metadata retained 30 days
  • transient edit inputs and masks retained 24 hours
  • terminal jobs can be manually deleted through the product API
• Link or mention the canonical product endpoint: GET /v2/retention-policy
• Fix stale image_generate examples to use the current asset_url shape returned by img-forge.
• If the gateway has or adds tool metadata/policy overlays for public tools, include a concise retention note for image_generate so clients and downstream docs can surface it.

Notes

This is documentation/contract propagation. The enforcement source of truth remains img-forge.