fix(keymap): a keybinding in config adds a chord instead of replacing the default (#868)

effective_bindings kept one chord per action and set_binding overwrote that
slot, so "NextTab": "cmd-shift-]" silently took Ctrl+Tab away.

A string in keybindings now adds a chord beside the action's default (or
preset) chord; "" still unbinds, as configs and the docs already rely on; a
list is the exact chord set, [] unbinds. Configured chords are installed after
every shipped one, so a chord the user names wins a tie with another action's
default. The Settings page lists every chord of an action, and recording a
shortcut writes the list shape (it sets the binding) and takes only the stolen
chord from the action that had it.

Claude-Session: https://claude.ai/code/session_01JRqYZ9E153WpSHGS2AW3BM

docs/customization/keybindings.mdx

@@ -39,13 +39,26 @@ the panel tabs. They are all in the command palette, and all bindable here.
 ```json
 {
   "keybindings": {
-    "SplitRight": "cmd-d",
+    "NextTab": "cmd-shift-]",
+    "SplitRight": ["cmd-d"],
     "ResizePaneLeft": "ctrl-alt-left",
-    "ToggleSftp": "cmd-shift-u"
+    "ToggleFullscreen": ""
   }
 }
 ```
 
+An action's value takes one of two shapes:
+
+| Value | Means |
+|---|---|
+| `"cmd-shift-]"` | **Add** this shortcut. The default keeps working — <kbd>⌃ ⇥</kbd> still switches tabs |
+| `["cmd-d"]` | **Replace**: exactly these shortcuts, nothing else. List several to have several |
+| `""` or `[]` | **Unbind** the action |
+
+Recording a shortcut on the Settings page replaces, so it writes the list
+shape. A shortcut you add that another action already uses wins: it is the one
+that runs.
+
 The syntax is modifiers joined by `-`, then the key. Chords are separated by a
 space.