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.

docs/constellation.md

@@ -30,16 +30,16 @@ use a flake to declare its tools without Nix owning the surrounding workspace.
 - Outlook is acceptable as an application, but its interaction and integration
   do not fit as naturally. Accessible mail for tools and agents matters separately
   from choosing a mail reader. Preserve meaning and source context, not just text.
-- Herdr provides useful structure: spaces, running agents and visible attention
-  states. Its keybindings and mouse-oriented interaction fit poorly. Agent-deck
-  gave a poor first impression through startup issues; no replacement is selected.
-  Explore whether Niri/Kitty or a terminal multiplexer can provide a more composable
-  workflow while retaining that understandable overview.
+- Herdr is the chosen agent workspace manager: its spaces, running-agent overview
+  and attention states fit. Adapt its keybindings to the keyboard-driven setup.
+  Use Ctrl+Alt for direct Herdr actions alongside Super for Niri and Ctrl navigation
+  in Neovim; retain Herdr's prefix bindings. Agent-deck's startup issues and other
+  session-manager alternatives are not the current focus.
 - Current friction is a lack of consistent project and agent working practices,
   including Niri workspace use. Tool choice and agent collaboration need to be
   considered together. Alignment, maintainability and human attention matter
-  alongside speed/cost; compare workflows through concrete tasks before adopting
-  a fixed delegation pattern.
+  alongside speed/cost. Agent-workflow evaluation is deferred; no fixed delegation
+  pattern has been selected.
 - Repositories should contain code and documentation needed by that code, not
   become containers for general notes and company material destined for Git remotes.
 - Company work includes repositories, reference material, notes, credentials and