vim normal-mode navigation in non-editor panes (T-406)

The structural T-403 child: make vim normal mode mean navigation in panes
that were mouse-only. The passive global key path can't run multi-chord
sequences (D-82), so each pane hosts its own SequenceMatcher — factored into
a reusable PaneKeyNav that resolves the live keymap and dispatches nav.*
intents while a pane holds focus under the vim preset.

- nav.* intents (down/up/pageDown/pageUp/top/bottom/expandOrRight/
  collapseOrLeft/activate) — preset-neutral; vim.yaml binds j/k/ctrl+d/ctrl+u/
  gg/G/l/h/[o,enter] under `vim.normal && !editor.focused`.
- The editor publishes an `editor.focused` scope flag from its focus node, so
  the same keys stay buffer motions while the editor is focused and become nav
  when a pane is — resolved by file order + the guard (no change to the editor
  motion bindings).
- File tree: a flattened visible-index selection cursor in FileTreeController
  (j/k move, h collapse-or-out, l expand-or-into, o/enter open), with a focus
  ring + scroll-into-view.
- Conversation: j/k line-scroll, ctrl+d/u half-page, gg top, G bottom — G
  re-arms follow-tail.

Foundation for T-404/T-405/T-407, which build on the per-pane matcher and the
editor.focused guard. Git panel + ticket board list nav deferred to a
follow-up (the ticket says lists can trail). Tests: keymap resolution under
both scopes, PaneKeyNav dispatch, the controller selection model, and
end-to-end key-driven nav in both panes.

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

assets/keymaps/vim.yaml

@@ -63,6 +63,43 @@ bindings:
   - intent: text.scaleReset
     keys: [ctrl+0, meta+0]
 
+  # ---- Pane navigation (non-editor panes) ------------------------------
+  # When a non-editor pane holds focus (file tree, conversation, lists), the
+  # same motion keys mean NAVIGATION, not buffer edits (T-406). The
+  # `!editor.focused` guard keeps these out of the editor's way; the editor
+  # publishes `editor.focused` while it has focus. These MUST precede the
+  # editor motions below — the resolver takes the first matching binding in
+  # file order, so with a pane focused (editor.focused false) nav wins, and
+  # with the editor focused the `!editor.focused` clause fails and the buffer
+  # motion below wins. Each pane runs its own SequenceMatcher (PaneKeyNav).
+  - intent: nav.down
+    keys: j
+    when: "vim.normal && !editor.focused"
+  - intent: nav.up
+    keys: k
+    when: "vim.normal && !editor.focused"
+  - intent: nav.pageDown
+    keys: ctrl+d
+    when: "vim.normal && !editor.focused"
+  - intent: nav.pageUp
+    keys: ctrl+u
+    when: "vim.normal && !editor.focused"
+  - intent: nav.top
+    keys: "g g" # gg
+    when: "vim.normal && !editor.focused"
+  - intent: nav.bottom
+    keys: shift+g # G
+    when: "vim.normal && !editor.focused"
+  - intent: nav.expandOrRight
+    keys: l
+    when: "vim.normal && !editor.focused"
+  - intent: nav.collapseOrLeft
+    keys: h
+    when: "vim.normal && !editor.focused"
+  - intent: nav.activate
+    keys: [o, enter]
+    when: "vim.normal && !editor.focused"
+
   # ---- Mode transitions ------------------------------------------------
   - intent: command:vim.mode.visual
     keys: v