docs(computer-use): document the headless i3 session

home/README.md

@@ -22,6 +22,7 @@ home/
 ├── linux/             # Linux-specific home manager configurations
 │   ├── base/          # Linux base configurations
 │   ├── gui/           # Linux GUI applications
+│   │   ├── i3/        # i3 — headless computer-use session (see i3/README.md)
 │   │   ├── niri/      # Niri window manager
 │   │   └── ...
 │   ├── editors/       # Linux-specific editors
@@ -42,6 +43,7 @@ home/
 
 2. **linux**: Linux-specific configuration
    - Desktop environments (Noctalia Shell, Niri compositor)
+   - Headless computer-use session (`gui/i3`, Xvfb + i3; see `gui/i3/README.md`)
    - Linux-specific GUI applications
    - System integration tools
 

home/linux/gui/i3/README.md

@@ -0,0 +1,39 @@
+# i3 — headless computer-use session
+
+This module provides the headless X11 session used by the computer-use VMs (`idols-ruby`,
+`idols-kana`). It is **not** a normal desktop configuration and is not meant to be used
+interactively by a person.
+
+## Purpose
+
+Give a computer-use agent a virtual 1920x1080 X display it can observe and control: screenshots,
+window discovery/activation, synthetic input, and the AT-SPI accessibility tree. The agent itself
+runs elsewhere and connects over MCP; this module is only the "computer" side.
+
+The session is built on Xvfb plus i3 because the drivers support native X11 at their highest tier
+(cua-driver "Linux X11 — Supported"; computer-use-linux has an i3 backend). cua-driver's Sway lane
+is XWayland/X11-based, so a native X11 session is simpler and better supported than Sway/Wayland.
+
+## How it is wired
+
+- `home/linux/gui/i3/default.nix` defines the option `modules.desktop.computerUse` and the user
+  services: `xvfb` → `i3` → (`x11vnc`, `cua-driver serve`).
+- `modules/nixos/desktop/computer-use.nix` provides the system side (`linger`, AT-SPI, fonts, Mesa,
+  Clash Verge proxy, proxy-region timezone).
+- `computer-use-init.sh` (exec'd by i3) publishes `DISPLAY`/`XDG_SESSION_TYPE` to the systemd user
+  manager, clears any stale `WAYLAND_DISPLAY`, and enables the AT-SPI bridge via a gsettings key.
+- Drivers live in `overlays/cua-driver.nix` and `overlays/computer-use-linux.nix`.
+- Host wiring: `home/hosts/linux/idols-ruby.nix`, `home/hosts/linux/idols-kana.nix`.
+
+## The i3 config
+
+The generated config is intentionally minimal:
+
+- `font pango:monospace 10`
+- `default_border normal` — keeps title bars, so the client area is slightly smaller than the
+  screen. That matches a real desktop and avoids a full-screen-sized viewport.
+- `exec --no-startup-id .../computer-use-init.sh` — one-time session setup, see above.
+
+There are no keybindings, bars or autostarted apps: agents drive the session through the drivers,
+not the keyboard. Manual inspection is done over VNC (`x11vnc`, bound to `127.0.0.1`; reach it
+through an SSH tunnel).