Add a native tmux Module and enable it on neogaia

Configure tmux through home-manager's programs.tmux: the settings it exposes
as options (prefix, keyMode, mouse, baseIndex, clock24, escapeTime,
historyLimit, terminal) are set as options, and every setting it has no option
for is read verbatim from modules/tmux/extra.conf. No tmux plugin manager is
used. The generated config is behaviourally identical to the reference, verified
by parsing it with a live tmux binary.

modules/tmux/extra.conf

@@ -0,0 +1,49 @@
+# Copy-mode selections land in the system clipboard via the terminal's own
+# OSC52 escape sequence.
+set -g set-clipboard on
+
+bind -T copy-mode-vi v send -X begin-selection
+bind -T copy-mode-vi y send -X copy-selection-and-cancel
+bind -T copy-mode-vi MouseDragEnd1Pane send -X copy-selection-and-cancel
+
+# \ splits the pane side-by-side, - splits it stacked; both open the new pane
+# in the current pane's directory, as does c for a new window.
+unbind %
+unbind '"'
+bind \\ split-window -h -c "#{pane_current_path}"
+bind - split-window -v -c "#{pane_current_path}"
+bind c new-window -c "#{pane_current_path}"
+
+bind h select-pane -L
+bind j select-pane -D
+bind k select-pane -U
+bind l select-pane -R
+
+set -g renumber-windows on
+
+bind r source-file ~/.config/tmux/tmux.conf \; display-message "tmux.conf reloaded"
+
+# Advertise RGB (true-color) capability for every terminal type.
+set -ag terminal-overrides ",*:RGB"
+
+# monitor-bell flags a background window in the status bar when it rings the
+# terminal bell while unfocused. bell-action=none suppresses the actual BEL
+# (no beep or flash); the per-window status highlight is independent of it.
+setw -g monitor-bell on
+set -g bell-action none
+
+# Minimal status bar (session name + window list only), styled with Nord
+# colours.
+set -g status-position bottom
+set -g status-style "bg=#2E3440,fg=#D8DEE9"
+set -g status-left " #S "
+set -g status-left-length 20
+set -g status-right ""
+setw -g window-status-current-style "bg=#88C0D0,fg=#2E3440,bold"
+setw -g window-status-current-format " #I:#W "
+setw -g window-status-format " #I:#W "
+setw -g window-status-style "fg=#4C566A"
+setw -g window-status-bell-style "bg=#BF616A,fg=#2E3440,bold"
+
+set -g pane-border-style "fg=#3B4252"
+set -g pane-active-border-style "fg=#88C0D0"