feat(copilot-chat): enhance settings and keymaps

Add detailed Copilot Chat settings including model selection, context
source, selection register, and highlight options. Expand keymaps to
support both normal and visual modes for toggling chat and prompts.
Improves usability and customization of Copilot Chat in Neovim.

config/plugins/utils/copilot-chat.nix

@@ -6,6 +6,12 @@
     settings = {
       # Model to use for Copilot Chat.
       model = "gpt-4.1";
+      # Use all buffers for context.
+      context = "buffers";
+      # Use yanked text for selection.
+      selection = "unnamed";
+      # Don't highlight the selection.
+      highlight_selection = false;
       # Window settings for Copilot Chat.
       window = {
         # Width of the chat window.
@@ -17,7 +23,7 @@
   # Keymaps for Copilot Chat.
   keymaps = [
     {
-      mode = ["n"];
+      mode = ["n" "v"];
       key = "<leader>ct";
       action = "<cmd>CopilotChatToggle<cr>";
       options = {
@@ -27,7 +33,7 @@
     }
 
     {
-      mode = ["n"];
+      mode = ["n" "v"];
       key = "<leader>cp";
       action = "<cmd>CopilotChatPrompts<cr>";
       options = {