docs: add hotfix-any-environment Mermaid diagrams

[joshua-temple]

Problem

The hotfix workflow is one of cascade's distinguishing capabilities (it targets any environment, not just the production tip), but neither the README nor the workflows reference page showed it visually. Readers had to reconstruct the roll-forward-first default, the per-environment integration branch, the resolution PR, and the rejoin path from prose alone.

Fix

• README gains a new ## Hotfix any environment section. One short paragraph plus a Mermaid diagram that lands a fix on staging (not prod) to emphasize that any environment is a valid target, shows the cherry-pick onto env/staging, the cascade-hotfix resolution PR, the staging-only deploy, the diverged -rc.N.hotfix.M state, and the rejoin once a trunk SHA containing the fix is promoted. Uses the existing teal/copper theme directive from the other README diagrams.
• The existing ## Hotfix section in the workflows reference gains a diagram at the top covering the fuller flow: roll forward first, integration branch, clean vs conflict resolution PR, finalize, diverged state, and rejoin under the patch-containment guard. No per-diagram init directive, so it inherits the docs site's global Mermaid theme. The surrounding prose is unchanged.

Verification

• All four Mermaid renders (both diagrams, light and dark backgrounds) compile cleanly with mermaid-cli.
• npm run build in docs/ succeeds; workflows/index.html regenerates without error.
• Documentation-only change; no code paths touched.