Establish project foundation: domain model, MVI spec, environment reference

Capture the design work for the NixOS migration before any implementation:

- .claude/CONTEXT.md: domain glossary (Host, Module, Skeleton, Auto-loader,
  Enable convention, unstable/stable overlay)
- .claude/adr/0001-sops-nix-for-secrets.md: secrets tooling decision
- .claude/spec/laptop-mvi.md: frozen minimum-viable-install spec for neogaia

- reference/: read-only snapshot of the current CachyOS configs (secrets and
  state excluded), plus ENVIRONMENT.md profiling the live environment to guide
  replication

reference/home/.config/tmux/tmux.conf

@@ -0,0 +1,74 @@
+# Prefix: Ctrl-Space. Chosen over Ctrl-b (awkward reach) and Ctrl-a (collides
+# with readline's beginning-of-line, which fights editing text in shells and
+# in Claude Code's prompt). Verified clear of IME/KDE/Claude Code bindings.
+unbind C-b
+set -g prefix C-Space
+bind C-Space send-prefix
+
+set -g mouse on
+# OSC52 lets copy-mode selections land in the system clipboard via the
+# terminal itself (Alacritty supports it) -- no wl-copy/xclip needed, and it
+# still works over SSH later since the escape sequence travels with the data.
+set -g set-clipboard on
+
+set -g mode-keys vi
+set -g status-keys vi
+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
+
+# tmux's -h/-v split flags name the *arrangement*, not the divider line, which
+# is backwards from how the divider looks -- so pick keys by what they draw:
+# \ draws a side-by-side split (vertical line), - draws a stacked split
+# (horizontal line). Unshifted versions of |/-  since splitting is frequent.
+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 base-index 1
+setw -g pane-base-index 1
+set -g renumber-windows on
+
+bind r source-file ~/.config/tmux/tmux.conf \; display-message "tmux.conf reloaded"
+
+# True color passthrough. ",*" (rather than naming Alacritty's xterm-256color
+# specifically) so this keeps working if the terminal emulator changes later.
+set -g default-terminal "tmux-256color"
+set -ag terminal-overrides ",*:RGB"
+
+# Default 500ms delay on Esc exists to disambiguate meta-key sequences; it
+# reads as noticeable lag exiting insert mode in neovim, so drop it.
+set -sg escape-time 10
+
+set -g history-limit 10000
+
+# Flag a background window in the status bar when its Claude Code session
+# rings the terminal bell (permission prompt / task done while unfocused).
+# bell-action=none stops tmux from ever passing the actual BEL through to
+# Alacritty (no beep, no flash) -- monitor-bell's per-window tracking for the
+# status-line highlight is independent of that and keeps working.
+setw -g monitor-bell on
+set -g bell-action none
+
+# Minimal status bar (session + window list only), styled to match the Nord
+# theme already used in alacritty.toml.
+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"