workspace tab cycle commands + ctrl+pagedown/up (T-405 part 1)

Add workspace.tab.next / workspace.tab.previous commands that cycle the
Slots.workspace tab strip with wraparound (no-op under two tabs), bound
ctrl+pagedown / ctrl+pageup across every preset via defaultBindings. Single-
chord, so no global matcher needed. Activating a tab also focuses the
workspace slot.

Part 2 (vim gt/gT) is deferred — it needs the global multi-chord matcher
T-404 introduces. T-405 stays open for that follow-up.

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

CHANGELOG.md

@@ -18,6 +18,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- **Workspace tab cycling with ctrl+pagedown / ctrl+pageup.** New
+  `workspace.tab.next` / `workspace.tab.previous` commands cycle the workspace
+  tab strip with wraparound, bound across every preset. (T-405)
 - **Vim normal-mode navigation works outside the editor.** Under the vim preset,
   a focused file tree or conversation now responds to j/k, ctrl+d/ctrl+u, gg/G,
   and (tree) h/l/o — a selection cursor in the tree, scrolling in the