PR plan-preview diagram hook in the generator

[joshua-temple]

Split from #307.

Scope: emit a rendered pipeline diagram into the PR plan-preview via the generator. The
preview surface already exists (PRPreviewConfig, cascade-pr-preview.yaml from
NewPRPreviewGenerator). Emit a diagram of the pending change into that read-only preview
lane, gated behind the existing pr-preview opt-in so manifests without the preview lane are
unaffected.

Touches internal/generate/command.go and needs its own e2e scenario per repo standards
(generator surface): a manifest with pr-preview enabled produces a preview workflow whose
output includes the diagram; a manifest without it is unchanged.

Acceptance:

• With pr-preview enabled, the PR preview includes a pipeline diagram of the change.
• A manifest without pr-preview is unchanged.
• Covered by an e2e scenario.