panel-to-panel focus traversal via F6 / Shift+F6 (T-105)

Each SlotHost now owns a FocusScopeNode and registers it with
FocusTracker on mount. The render is wrapped in
FocusScope + FocusTraversalGroup so Tab stays within a panel and
slot-level focus is observable.

When a slot's scope gains focus, SlotHost pushes
(slot, activeContributionId) to FocusTracker — this collapses the
parallel-tracker model the consultant flagged. FocusTracker keeps
its setActive surface for explicit callers (palette, etc.) but
slot-scoped tab activation feeds it automatically.

Two new intents, two new bindings:
  FocusNextPanelIntent     → F6
  FocusPreviousPanelIntent → Shift+F6
(VS Code convention; preset YAML.)

The cycle skips slots without a registered scope, so a layout that
hides the context panel doesn't strand focus on a missing target.
Fewer than two registered → no-op.

SlotHost split into a stateful outer (scope + registry) and a
stateless `_SlotBody` (the existing slot-specific rendering),
keeping the build straightforward.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

assets/keymaps/default.yaml

@@ -30,6 +30,13 @@ bindings:
   - intent: focus.previous
     keys: shift+tab
 
+  # Panel-to-panel cycling (sidebar → workspace → context).
+  # VS Code convention.
+  - intent: focus.nextPanel
+    keys: f6
+  - intent: focus.previousPanel
+    keys: shift+f6
+
   # -- Command palette --------------------------------------------------
   - intent: palette.open
     keys: [ctrl+shift+p, meta+shift+p]