fix(formatter): get more correct markdown types

Instead of just using the filename extension, use the neovim filetype
(with some specific overrides). Fall back to the filename extension.

This means we render our markdown sections with `markdown` as the type
instead of `md` and that enables rendering embedded codefences
correctly.

Hopefully the last fix on this :)

Author: cameronr

lua/opencode/context.lua

@@ -269,14 +269,14 @@ end
 
 ---@param selection OpencodeContextSelection
 local function format_selection_part(selection)
-  local lang = selection.file and selection.file.extension or ''
+  local lang = util.get_markdown_filetype(selection.file and selection.file.name or '') or ''
 
   return {
     type = 'text',
     text = vim.json.encode({
       context_type = 'selection',
       file = selection.file,
-      content = string.format('```%s\n%s\n```', lang, selection.content),
+      content = string.format('`````%s\n%s\n`````', lang, selection.content),
       lines = selection.lines,
     }),
     synthetic = true,

lua/opencode/ui/formatter.lua

@@ -428,7 +428,7 @@ end
 ---@param metadata FileToolMetadata Metadata for the tool use
 function M._format_file_tool(output, tool_type, input, metadata)
   local file_name = input and vim.fn.fnamemodify(input.filePath, ':t') or ''
-  local file_type = input and vim.fn.fnamemodify(input.filePath, ':e') or ''
+  local file_type = input and util.get_markdown_filetype(input.filePath) or ''
   local tool_action_icons = { read = icons.get('read'), edit = icons.get('edit'), write = icons.get('write') }
 
   M._format_action(output, tool_action_icons[tool_type] .. ' ' .. tool_type, file_name)

lua/opencode/util.lua

@@ -389,4 +389,30 @@ function M.check_prompt_allowed(guard_callback, mentioned_files)
   return result, nil
 end
 
+--- Get the markdown type to use based on the filename. First gets the neovim type
+--- for the file. Then apply any specific overrides. Falls back to using the file
+--- extension if nothing else matches
+--- @param filename string filename, possibly including path
+--- @return string markdown_filetype
+function M.get_markdown_filetype(filename)
+  local file_type_overrides = {
+    javascriptreact = 'jsx',
+    typescriptreact = 'tsx',
+    sh = 'bash',
+    yaml = 'yml',
+  }
+
+  local file_type = vim.filetype.match({ filename = filename }) or ''
+
+  if file_type_overrides[file_type] then
+    return file_type_overrides[file_type]
+  end
+
+  if file_type and file_type ~= '' then
+    return file_type
+  end
+
+  return vim.fn.fnamemodify(filename, ':e')
+end
+
 return M