fix(parser): detect filetype from file content (shebang/modeline)

When a file has no extension but contains a shebang (e.g., `#!/bin/bash`),
filetype detection now reads the first 10 lines from disk and uses
`vim.filetype.match({ filename, contents })` for content-based detection.

This enables syntax highlighting for files like `build` scripts that rely
on shebang detection, even when the file isn't open in a buffer.

Detection order:
1. Existing buffer's filetype (already implemented in #69)
2. File content (shebang/modeline) - NEW
3. Filename extension only

Also adds `filetype on` to test helpers to ensure `vim.g.ft_ignore_pat`
is set, which is required for shell detection.

spec/helpers.lua

@@ -2,6 +2,8 @@ local plugin_dir = vim.fn.getcwd()
 vim.opt.runtimepath:prepend(plugin_dir)
 vim.opt.packpath = {}
 
+vim.cmd('filetype on')
+
 local function ensure_parser(lang)
   local ok = pcall(vim.treesitter.language.inspect, lang)
   if not ok then

spec/parser_spec.lua

@@ -359,5 +359,67 @@ describe('parser', function()
       delete_buffer(file_buf)
       delete_buffer(diff_buf)
     end)
+
+    it('detects filetype from file content shebang without open buffer', function()
+      local repo_root = '/tmp/diffs-test-shebang'
+      vim.fn.mkdir(repo_root, 'p')
+
+      local file_path = repo_root .. '/build'
+      local f = io.open(file_path, 'w')
+      f:write('#!/bin/bash\n')
+      f:write('set -e\n')
+      f:write('echo "hello"\n')
+      f:close()
+
+      local diff_buf = create_buffer({
+        'M build',
+        '@@ -1,2 +1,3 @@',
+        ' #!/bin/bash',
+        '+set -e',
+        ' echo "hello"',
+      })
+      vim.api.nvim_buf_set_var(diff_buf, 'diffs_repo_root', repo_root)
+
+      local hunks = parser.parse_buffer(diff_buf)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('build', hunks[1].filename)
+      assert.are.equal('sh', hunks[1].ft)
+
+      delete_buffer(diff_buf)
+      os.remove(file_path)
+      vim.fn.delete(repo_root, 'rf')
+    end)
+
+    it('detects python from shebang without open buffer', function()
+      local repo_root = '/tmp/diffs-test-shebang-py'
+      vim.fn.mkdir(repo_root, 'p')
+
+      local file_path = repo_root .. '/deploy'
+      local f = io.open(file_path, 'w')
+      f:write('#!/usr/bin/env python3\n')
+      f:write('import sys\n')
+      f:write('print("hi")\n')
+      f:close()
+
+      local diff_buf = create_buffer({
+        'M deploy',
+        '@@ -1,2 +1,3 @@',
+        ' #!/usr/bin/env python3',
+        '+import sys',
+        ' print("hi")',
+      })
+      vim.api.nvim_buf_set_var(diff_buf, 'diffs_repo_root', repo_root)
+
+      local hunks = parser.parse_buffer(diff_buf)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('deploy', hunks[1].filename)
+      assert.are.equal('python', hunks[1].ft)
+
+      delete_buffer(diff_buf)
+      os.remove(file_path)
+      vim.fn.delete(repo_root, 'rf')
+    end)
   end)
 end)