feat: toggle review in a dedicated tab

[quantk]

What changed

• open new reviews directly in a dedicated Herdr tab instead of a split
• make F6 switch between the persistent review tab and the exact source agent tab
• migrate an already-running legacy split review into a dedicated tab without restarting it
• name new and existing review tabs Review explicitly
• serialize and coalesce overlapping UI resize renders to prevent the initial compressed layout
• update README, agent guide, manifest description, and integration/UI tests

Why

The split/background-pane toggle made review layout less predictable. Dedicated tabs provide the full terminal width and a symmetric F6 workflow. New tabs can also emit multiple startup resize events; serialized render passes prevent those events from temporarily corrupting the visible layout.

Validation

• npm run version:check
• npm test (43 Node tests, 14 Bun UI tests)
• npm run check