Add custom Credo check for conn shadowing in tests

Detects `conn = get(conn, ...)` patterns and directs to
refactor_conn_aliasing.sh for automatic fixing.

app/lib/firehose/checks/no_conn_shadowing.ex

@@ -0,0 +1,44 @@
+defmodule Firehose.Checks.NoConnShadowing do
+  use Credo.Check,
+    base_priority: :normal,
+    category: :readability,
+    explanations: [
+      check: """
+      Conn shadowing (`conn = get(conn, ...)`) makes Phoenix controller tests
+      noisy. Use pipe chains instead:
+
+          body = conn |> get("/path") |> html_response(200)
+
+      Run `./refactor_conn_aliasing.sh <file>` to fix automatically.
+      """
+    ]
+
+  @http_verbs ~w(get post put patch delete head options)a
+
+  @impl true
+  def run(%SourceFile{} = source_file, params) do
+    issue_meta = IssueMeta.for(source_file, params)
+
+    source_file
+    |> Credo.Code.prewalk(&traverse(&1, &2, issue_meta))
+    |> Enum.reverse()
+  end
+
+  defp traverse({:=, meta, [{:conn, _, _}, {verb, _, [{:conn, _, _} | _]}]} = ast, issues, issue_meta)
+       when verb in @http_verbs do
+    issue = issue_for(issue_meta, meta[:line], verb)
+    {ast, [issue | issues]}
+  end
+
+  defp traverse(ast, issues, _issue_meta) do
+    {ast, issues}
+  end
+
+  defp issue_for(issue_meta, line_no, verb) do
+    format_issue(
+      issue_meta,
+      message: "Conn shadowing detected (`conn = #{verb}(conn, ...)`). Run `./refactor_conn_aliasing.sh <file>` to fix.",
+      line_no: line_no
+    )
+  end
+end