Two additive coordinator affordances:

- dismissModal(): presenter-side counterpart of present(_:as:), on every
  coordinator type. Removes the top modal and fires its onDismiss once,
  including view-only modals with no child coordinator. On a
  FlowCoordinatable it removes only the topmost modal and is a safe no-op
  when nothing is presented.
- shouldSelect(tab:isReselection:) on TabCoordinatable: intercepts
  UI-driven tab taps to guard/redirect/pop-to-root on re-tap. Redirects
  from inside the hook are preserved; programmatic selection bypasses it.
  Defaults to true — purely additive.

Docs (AGENTS.md, web changelog) and tests updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>