feat: add AppAction enum and Doom Emacs keybinding configuration

18-variant AppAction enum + build_normal_keymap() with hjkl, gg/G, gt/gT,
1-6 tabs, C-d/C-u, SPC leader tree (buffer/help/quit/notifications).

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/claude-agent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/input/action.rs

@@ -0,0 +1,25 @@
+#[derive(Clone, Debug)]
+pub enum AppAction {
+    // Movement
+    MoveUp,
+    MoveDown,
+    FocusLeft,
+    FocusRight,
+    CycleFocus,
+    GotoFirst,
+    GotoLast,
+    HalfPageDown,
+    HalfPageUp,
+
+    // Tabs
+    NextTab,
+    PrevTab,
+    GotoTab(usize),
+
+    // System
+    Quit,
+    Refresh,
+    ShowHelp,
+    ToggleNotifications,
+    Escape,
+}