agent: Add item to open Prompt Library in the panel's menu (#28877)

Release Notes:

- agent: Added a menu item to open the Prompt Library from the panel's
dropdown menu on the top right.

assets/keymaps/default-linux.json

@@ -630,6 +630,7 @@
       "ctrl-alt-n": "agent::NewTextThread",
       "ctrl-shift-h": "agent::OpenHistory",
       "ctrl-alt-c": "agent::OpenConfiguration",
+      "ctrl-alt-p": "assistant::OpenPromptLibrary",
       "ctrl-i": "agent::ToggleProfileSelector",
       "ctrl-alt-/": "assistant::ToggleModelSelector",
       "ctrl-shift-a": "agent::ToggleContextPicker",

assets/keymaps/default-macos.json

@@ -286,6 +286,7 @@
       "cmd-alt-n": "agent::NewTextThread",
       "cmd-shift-h": "agent::OpenHistory",
       "cmd-alt-c": "agent::OpenConfiguration",
+      "cmd-alt-p": "assistant::OpenPromptLibrary",
       "cmd-i": "agent::ToggleProfileSelector",
       "cmd-alt-/": "assistant::ToggleModelSelector",
       "cmd-shift-a": "agent::ToggleContextPicker",