`444c75c10a` rendered `titleBar.center` in the workspace panel header on
full pages so the kanban board switcher would sit beside the page title
instead of colliding with the sidebar tab strip. That made the area migrate
between two React subtrees on chat <-> page navigation: the old instance's
effect cleanup ran after the new instance's setup and wiped every global
side effect a third-party plugin had just re-created (#114290).
Keep both intents: `titleBar.center` is a permanent titlebar slot again (the
salvaged commit), and page-owned controls move to a dedicated
`WORKSPACE_PAGE_HEADER_AREA` (`workspace.pageHeader`) that the workspace
pane projects into its vetoed tab row while `$workspaceIsPage` holds —
exactly the placement `444c75c10a` introduced, just not through the
plugin-facing titlebar area. Kanban's board switcher contributes there.
- SDK exports `WORKSPACE_PAGE_HEADER_AREA`; `TITLEBAR_AREAS` documents the
permanent-mount contract.
- Test: a `titleBar.center` component's effect runs setup once and cleanup
never across a chat -> /skills -> chat round trip (red on base).
- Docs: plugin SDK guide covers the lifecycle contract and the page-header
area.
14d862384b