Update the docs and add AGENTS.md

CONFIG.md

@@ -174,6 +174,9 @@ project's own menu/keyboard chrome text (title bar, buttons, etc.).
   open. See README.md section 3 ("Game assets") for where `mfdfrn.res`/
   `mfdger.res` come from - the GOG Enhanced Edition installer alone doesn't
   include them.
+- The intro cutscene is also played in the selected language when its
+  voice-over file (`svfrintr.res`/`svgeintr.res` under `res/data/`) is
+  present; otherwise the English intro plays.
 
 ## Player Position (`player_position`)
 
@@ -230,6 +233,9 @@ wait 8 seconds, to cancel without changing it.
 - **Restart required?**: No (live-appliable) - a rebind takes effect
   immediately, and persists across restarts once saved (**Save
   Configuration**, same as every other group here).
+- **Menu movement**: while the Controller Options tab is open, the menu's own
+  thumbstick snap-turn and smooth-move (see `player_position`) are disabled, so
+  pressing inputs to assign them doesn't move you away from the screen.
 - **Not included here**: opening/closing the VR menu itself is a fixed,
   non-remappable button (see "Menu button" below) - there's no
   `menu_toggle` field, and the Controller Options tab never lets you
@@ -358,7 +364,9 @@ keyboard (there is no hold behavior).
 - `keypad 0`-`keypad 9`: a controller button per digit is not useful; use the
   on-screen keyboard.
 - `toggle_freelook` (F): the free-view actions cover mouse look.
-- The 4 `cheat_*` entries: debug cheats. They are on the menu's Cheats tab instead.
+- The 4 `cheat_*` entries: debug cheats. They are on the menu's Cheats tab
+  instead, along with four switches for unlimited health, energy, ammo and
+  items. Those are not saved to the config.
 - `mac_help` (disabled) and `audiolog_cancel` (its build flag is not
   defined): they are not real hotkeys in this build.
 - `toggle_opengl` (Ctrl+G): the Video Options tab's Renderer buttons already