docs: improve README with better examples and cleaner structure

Previously, the documentation for custom prompts, system prompts and
contexts was scattered and duplicated across multiple sections. This
change consolidates and improves the documentation by:

- Moving examples next to their relevant sections
- Adding clearer and more concise examples for custom prompts, system
  prompts and contexts
- Removing duplicate content that was spread across multiple sections
- Making github/copilot.vim the primary recommended Copilot backend

Signed-off-by: Tomas Slusny <[email protected]>

Author: deathbeam

README.md

@@ -32,7 +32,7 @@ return {
     "CopilotC-Nvim/CopilotChat.nvim",
     branch = "canary",
     dependencies = {
-      { "zbirenbaum/copilot.lua" }, -- or github/copilot.vim
+      { "github/copilot.vim" }, -- or zbirenbaum/copilot.lua
       { "nvim-lua/plenary.nvim" }, -- for curl, log wrapper
     },
     build = "make tiktoken", -- Only on MacOS or Linux
@@ -52,7 +52,7 @@ Similar to the lazy setup, you can use the following configuration:
 
 ```vim
 call plug#begin()
-Plug 'zbirenbaum/copilot.lua'
+Plug 'github/copilot.vim'
 Plug 'nvim-lua/plenary.nvim'
 Plug 'CopilotC-Nvim/CopilotChat.nvim', { 'branch': 'canary' }
 call plug#end()
@@ -72,7 +72,7 @@ EOF
 mkdir -p ~/.config/nvim/pack/copilotchat/start
 cd ~/.config/nvim/pack/copilotchat/start
 
-git clone https://github.com/zbirenbaum/copilot.lua
+git clone https://github.com/github/copilot.vim
 git clone https://github.com/nvim-lua/plenary.nvim
 
 git clone -b canary https://github.com/CopilotC-Nvim/CopilotChat.nvim
@@ -121,6 +121,21 @@ Default prompts are:
 - `Tests` - Please generate tests for my code
 - `Commit` - Write commit message for the change with commitizen convention
 
+You can define custom prompts like this (only `prompt` is required):
+
+```lua
+{
+  prompts = {
+    MyCustomPrompt = {
+      prompt = 'Explain how it works.',
+      system_prompt = 'You are very good at explaining stuff',
+      mapping = '<leader>ccmc',
+      description = 'My custom prompt description',
+    }
+  }
+}
+```
+
 ### System Prompts
 
 System prompts specify the behavior of the AI model. You can reference system prompts with `/PROMPT_NAME` in chat.
@@ -131,6 +146,18 @@ Default system prompts are:
 - `COPILOT_REVIEW` - On top of the base instructions adds code review behavior with instructions on how to generate diagnostics
 - `COPILOT_GENERATE` - On top of the base instructions adds code generation behavior, with predefined formatting and generation rules
 
+You can define custom system prompts like this (works same as `prompts` so you can combine prompt and system prompt definitions):
+
+```lua
+{
+  prompts = {
+    Yarrr = {
+      system_prompt = 'You are fascinated by pirates, so please respond in pirate speak.',
+    }
+  }
+}
+```
+
 ### Sticky Prompts
 
 You can set sticky prompt in chat by prefixing the text with `> ` using markdown blockquote syntax.  
@@ -186,6 +213,39 @@ Default contexts are:
 - `files` - Includes all non-hidden filenames in the current workspace in chat context. Supports input.
 - `git` - Includes current git diff in chat context (default unstaged). Supports input.
 
+You can define custom contexts like this:
+
+```lua
+{
+  contexts = {
+    birthday = {
+      input = function(callback)
+        vim.ui.select({ 'user', 'napoleon' }, {
+          prompt = 'Select birthday> ',
+        }, callback)
+      end,
+      resolve = function(input)
+        input = input or 'user'
+        local birthday = input
+        if input == 'user' then
+          birthday = birthday .. ' birthday is April 1, 1990'
+        elseif input == 'napoleon' then
+          birthday = birthday .. ' birthday is August 15, 1769'
+        end
+        
+        return {
+          {
+            content = birthday,
+            filename = input .. '_birthday',
+            filetype = 'text',
+          }
+        }
+      end
+    }
+  }
+}
+```
+
 ### API
 
 ```lua
@@ -396,66 +456,6 @@ Also see [here](/lua/CopilotChat/config.lua):
 }
 ```
 
-For further reference, you can view @jellydn's [configuration](https://github.com/jellydn/lazy-nvim-ide/blob/main/lua/plugins/extras/copilot-chat-v2.lua).
-
-### Defining a prompt with command and keymap
-
-This will define prompt that you can reference with `/MyCustomPrompt` in chat, call with `:CopilotChatMyCustomPrompt` or use the keymap `<leader>ccmc`.
-It will use visual selection as default selection. If you are using `lazy.nvim` and are already lazy loading based on `Commands` make sure to include the prompt
-commands and keymaps in `cmd` and `keys` respectively.
-
-```lua
-{
-  prompts = {
-    MyCustomPrompt = {
-      prompt = 'Explain how it works.',
-      mapping = '<leader>ccmc',
-      description = 'My custom prompt description',
-      selection = require('CopilotChat.select').visual,
-    },
-  },
-}
-```
-
-### Referencing system or user prompts
-
-You can reference system or user prompts in your configuration or in chat with `/PROMPT_NAME` slash notation.
-For collection of default `COPILOT_` (system) and `USER_` (user) prompts, see [here](/lua/CopilotChat/prompts.lua).
-
-```lua
-{
-  prompts = {
-    MyCustomPrompt = {
-      prompt = '/COPILOT_EXPLAIN Explain how it works.',
-    },
-    MyCustomPrompt2 = {
-      prompt = '/MyCustomPrompt Include some additional context.',
-    },
-  },
-}
-```
-
-### Custom system prompts
-
-You can define custom system prompts by using `system_prompt` property when passing config around.
-
-```lua
-{
-  system_prompt = 'Your name is Github Copilot and you are a AI assistant for developers.',
-  prompts = {
-    Johnny = {
-      system_prompt = 'Your name is Johny Microsoft and you are not an AI assistant for developers.',
-      prompt = 'Explain how it works.',
-    },
-    Yarrr = {
-      system_prompt = 'You are fascinated by pirates, so please respond in pirate speak.'
-    },
-  },
-}
-```
-
-To use any of your custom prompts, simply do `:CopilotChat<prompt name>`. E.g. `:CopilotChatJohnny` or `:CopilotChatYarrr What is a sorting algo?`. Tab autocomplete will help you out.
-
 ### Customizing buffers
 
 You can set local options for the buffers that are created by this plugin: `copilot-diff`, `copilot-system-prompt`, `copilot-user-selection`, `copilot-chat`.

lua/CopilotChat/init.lua

@@ -570,6 +570,9 @@ function M.ask(prompt, config)
     local context_name = table.remove(split, 1)
     local context_input = table.concat(split, ':')
     local context_value = config.contexts[context_name]
+    if vim.trim(context_input) == '' then
+      context_input = nil
+    end
 
     if context_value then
       for _, embedding in ipairs(context_value.resolve(context_input, state.source)) do