feat: add quickfix list context support

This commit adds support for the quickfix list context in CopilotChat. The
quickfix context allows including the contents of files listed in the
quickfix list as part of the chat context.

The implementation includes:
- New quickfix context type in config.lua
- Function to extract and process quickfix list files
- Updated documentation in README.md with the new context type

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Author: deathbeam

README.md

@@ -262,6 +262,7 @@ Default contexts are:
   - `git:staged` - Includes staged changes in chat context.
 - `url` - Includes content of provided URL in chat context. Supports input.
 - `register` - Includes contents of register in chat context. Supports input (default +, e.g clipboard).
+- `quickfix` - Includes quickfix list file contents in chat context.
 
 You can define custom contexts like this:
 
@@ -487,6 +488,9 @@ Also see [here](/lua/CopilotChat/config.lua):
     register = {
       -- see config.lua for implementation
     },
+    quickfix = {
+      -- see config.lua for implementation
+    },
   },
 
   -- default prompts

lua/CopilotChat/config.lua

@@ -283,6 +283,12 @@ return {
         }
       end,
     },
+    quickfix = {
+      description = 'Includes quickfix list file contents in chat context.',
+      resolve = function()
+        return context.quickfix()
+      end,
+    },
   },
 
   -- default prompts

lua/CopilotChat/context.lua

@@ -282,6 +282,8 @@ end
 ---@param filetype string
 ---@return CopilotChat.context.embed?
 local function get_file(filename, filetype)
+  notify.publish(notify.STATUS, 'Reading file ' .. filename)
+
   local modified = utils.file_mtime(filename)
   if not modified then
     return nil
@@ -377,8 +379,6 @@ function M.file(filename)
     return nil
   end
 
-  notify.publish(notify.STATUS, 'Reading file ' .. filename)
-
   async.util.scheduler()
   local ft = utils.filetype(filename)
   if not ft then
@@ -532,6 +532,46 @@ function M.register(register)
   }
 end
 
+--- Get the content of the quickfix list
+---@return table<CopilotChat.context.embed>
+function M.quickfix()
+  async.util.scheduler()
+
+  local items = vim.fn.getqflist()
+  if not items or #items == 0 then
+    return {}
+  end
+
+  local unique_files = {}
+  for _, item in ipairs(items) do
+    local filename = item.filename or vim.api.nvim_buf_get_name(item.bufnr)
+    if filename then
+      unique_files[filename] = true
+    end
+  end
+
+  local files = vim.tbl_filter(
+    function(file)
+      return file.ft ~= nil
+    end,
+    vim.tbl_map(function(file)
+      return {
+        name = utils.filepath(file),
+        ft = utils.filetype(file),
+      }
+    end, vim.tbl_values(unique_files))
+  )
+
+  local out = {}
+  for _, file in ipairs(files) do
+    local file_data = get_file(file.name, file.ft)
+    if file_data then
+      table.insert(out, file_data)
+    end
+  end
+  return out
+end
+
 --- Filter embeddings based on the query
 ---@param copilot CopilotChat.Copilot
 ---@param prompt string