docs(niri): fold AGENTS.md overwrite into niri KB section

The working tree had replaced the operating contract in AGENTS.md with a
niri config-reload + keybinding-constraints note. Content relocated to its
proper home: tips.md 3-config-reload (SIGHUP footgun, load-config-file) and
key-bindings-syntax.md constraints section. AGENTS.md restored from HEAD.

omp-docs/tools/niri/key-bindings-syntax.md

@@ -54,6 +54,12 @@ wev
 
 Shifted keys require Shift + the unshifted key (for example, `Mod+Shift+Comma` for `<` on US QWERTY).
 
+## Constraints
+
+- One action per keybind. Each `{ ... }` block holds exactly one action; multiple actions like `{ move-window-up; consume-or-expel-window-up; }` are rejected at parse time.
+- No duplicate keys. The same key combo (e.g. `Mod+CTRL+Left`) can only appear once in `binds {}`. Adding a new binding for an existing key causes a parse error.
+- Valid consume/expel actions: `consume-or-expel-window-left`, `consume-or-expel-window-right`, and `expel-window-from-column`. The `consume-or-expel-window-up`/`-down` variants do NOT exist.
+
 ## Related
 
 * [Default Key Bindings](/tools/niri/default-bindings.md) — What niri ships with out of the box