visualizer: integration hooks (README diagram, PR plan-preview diagram, docs)

[joshua-temple]

Problem

The visualizer is most useful where people already look: a repo's README and a pull request.
Wire the rendered diagram into both, and document the feature.

Scope

1. README diagram - a committable artifact. The user runs cascade graph and commits the
   Mermaid into their README; GitHub renders it natively. For v1 the baseline is a stable
   --format mermaid stdout that the user pastes/commits. An in-place marker-block update mode
   (rewriting a fenced ```mermaid block between sentinel comments so the diagram can be
   refreshed) is a follow-up, kept opt-in and non-destructive.

2. PR plan-preview diagram (optional) - emit a diagram of what the pending change will do
   into the read-only PR plan-preview lane. The preview surface already exists: PRPreviewConfig
   (feat: optional PR plan-preview workflow #40) and the generated cascade-pr-preview.yaml via NewPRPreviewGenerator
   (internal/generate/command.go). The default surface is the GitHub job step-summary so a
   reviewer sees the change's shape (the exact surface is left flexible: step-summary, PR
   comment, or job-summary artifact). Gate it behind the existing pr-preview opt-in so manifests
   that do not use the preview lane are unaffected.

3. Docs page - a page under docs/src/content/docs/ documenting cascade graph: the
   granularities, the --format/--theme flags, the README workflow, and the PR-preview
   integration. Include a rendered Mermaid example.

This may be split (README + docs as one issue, PR-preview as a follow-up) since the PR-preview
hook depends on touching the generator and warrants its own e2e.

Tests (TDD)

• README marker-block update (if implemented): given a README with the sentinel block, the
  command rewrites only that block, idempotently; a README without the block is untouched.
• PR-preview integration: a manifest with pr_preview enabled produces a preview workflow
  whose output includes the diagram; a manifest without it is unchanged. Cover with an e2e/
  scenario (generator surface).
• Docs page builds (the docs site's existing check).

Acceptance

• A user can commit a cascade graph Mermaid diagram into a README and refresh it.
• With pr-preview enabled, the PR preview includes a pipeline diagram of the change.
• The docs page documents the command and renders an example.

Dependencies

#302, #303.