Explain which menu bar items can be hidden

Hiding other apps' icons worked, but Apple pins Spotlight, Clock, Battery
and Control Center to the right of the menu bar, so a third-party item can
never be placed after them to push them away. Document the real limit so
the behaviour is not mistaken for a bug.

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AGENTS.md

@@ -112,6 +112,19 @@ no control over this, so the app creates its own `NSStatusItem` and:
 `/tmp/simpletopbar-items.txt`; use it to verify the icon is not under the notch
 (x should be ~1200, not ~765).
 
+## Hiding other menu bar items
+
+`MenuBarHider` creates a zero-width spacer `NSStatusItem` at launch and grows its
+length to `hiddenLength` (10_000) to push the icons on its left off-screen.
+Verified with `--diag-hide`:
+
+- All third-party apps' icons go to x < 0 (cmux, Kapture, moomoo, WireGuard,
+  TextInputMenuAgent, Now Playing) and so does Wi-Fi.
+- **Spotlight, Clock, Battery and Control Center stay**: macOS pins them to the
+  far right, and a third-party item can never be inserted to their right, so the
+  spacer has nothing to push them with. Do not try to "fix" this; it is a macOS
+  constraint, not a bug.
+
 ## Gotchas
 
 - `AppListModel` is `@MainActor`; the `NSWorkspace` observers hop back with