feat: align Herdr keyboard navigation with the desktop

Add direct Ctrl+Alt pane, workspace, picker and agent navigation while retaining prefix shortcuts. Manage the existing theme and sidebar preferences with the keymap in Home Manager. Record Herdr as the chosen manager and defer agent evaluation.

Formatting, rendered TOML parsing, flake evaluation, Zenith system build and whitespace checks passed. User activation and interactive keyboard acceptance remain; no live sessions were changed.

modules/capabilities/ai.nix

@@ -120,6 +120,41 @@ in
         pkgs.ctx7
       ];
 
+      xdg.configFile."herdr/config.toml".text = ''
+        onboarding = false
+
+        [theme]
+        name = "kanagawa"
+        auto_switch = false
+
+        [ui]
+        agent_panel_sort = "spaces"
+
+        [ui.toast]
+        delivery = "off"
+
+        [keys]
+        # Super belongs to Niri; Ctrl navigation belongs to Neovim.
+        focus_pane_left = ["prefix+h", "ctrl+alt+h"]
+        focus_pane_down = ["prefix+j", "ctrl+alt+j"]
+        focus_pane_up = ["prefix+k", "ctrl+alt+k"]
+        focus_pane_right = ["prefix+l", "ctrl+alt+l"]
+        next_workspace = "ctrl+alt+u"
+        previous_workspace = "ctrl+alt+i"
+        navigate_workspace_down = "u"
+        navigate_workspace_up = "i"
+        workspace_picker = ["prefix+w", "ctrl+alt+o"]
+        goto = ["prefix+g", "ctrl+alt+g"]
+        focus_agent = "ctrl+alt+1..9"
+        previous_tab = ["prefix+p", "ctrl+alt+["]
+        next_tab = ["prefix+n", "ctrl+alt+]"]
+        new_tab = ["prefix+c", "ctrl+alt+enter"]
+        split_vertical = ["prefix+v", "ctrl+alt+v"]
+        split_horizontal = ["prefix+minus", "ctrl+alt+minus"]
+        zoom = ["prefix+z", "ctrl+alt+f"]
+        detach = ["prefix+q", "ctrl+alt+q"]
+      '';
+
       home.file.".codex/AGENTS.md".text = ''
         # Environment