fix(gep-validator): clean up Credo --strict findings after GEP-12 flip

GEP-12 commit 3dc1f9a enabled `Credo.Check.Warning.UnsafeToAtom` but
left 7 pre-existing Credo findings in the GEP validator that hadn't
tripped the previous disabled config. Most Credo categories exit
non-zero individually under --strict (refactor-opportunities = exit
code 8), which GitHub Actions treats as a failing step even when the
local `mix credo --strict` exits 0. CI went red on e3122f7.

Findings fixed:

lib/gep/validator.ex
- `unless X do ... else ...` → `if X do ... else ...`
  (2 sites: check_history_entries :314, check_readme_index :589).
- Nested-too-deep reduce bodies (2 sites in check_bidirectional)
  extracted into `check_superseded_by/4` and `check_back_ref/4`
  helpers. Added `nil`-head clauses for the "referenced GEP doesn't
  exist in number_map" case so the call site doesn't carry the
  nil-check.
- Credo's "avoid negated conditions in if-else" flagged `not in
  back_refs` — inverted the branches, kept the detail-building arm
  as the else clause.

test/gep/validator_test.exs
- `Enum.map(coll, f) |> Enum.join(sep)` → `Enum.map_join(coll, sep, f)`
  at 3 call sites in `build_frontmatter/1`. Zero semantic change.

Verification
------------
- `mix format --check-formatted` — clean.
- `mix compile --warnings-as-errors` — clean.
- `mix credo --strict` — exit 0, 0 findings. (Was exit 8 before.)
- `mix test --exclude inotify` — 588 tests, 0 failures.
- `mix test test/gep/validator_test.exs` — 27 tests, 0 failures.
- `mix gep.validate` — all 12 GEPs green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: foobarto

lib/gep/validator.ex

@@ -311,7 +311,9 @@ defmodule Gep.Validator do
             entry_errors ++
               [%{severity: :error, idx: idx, detail: "history[#{idx}] missing date"}]
           else
-            unless Regex.match?(@date_re, to_string(entry["date"])) do
+            if Regex.match?(@date_re, to_string(entry["date"])) do
+              entry_errors
+            else
               entry_errors ++
                 [
                   %{
@@ -320,8 +322,6 @@ defmodule Gep.Validator do
                     detail: "history[#{idx}] invalid date format: #{entry["date"]}"
                   }
                 ]
-            else
-              entry_errors
             end
           end
 
@@ -492,54 +492,14 @@ defmodule Gep.Validator do
         acc =
           (record.supersedes || [])
           |> Enum.reduce(acc, fn ref_n, acc ->
-            ref = number_map[ref_n]
-
-            if ref == nil do
-              acc
-            else
-              if ref.superseded_by != record.number do
-                acc ++
-                  [
-                    %{
-                      severity: :error,
-                      label: "Bidirectional links",
-                      detail:
-                        "GEP-#{gep_label(record.number)} supersedes GEP-#{gep_label(ref_n)} but GEP-#{gep_label(ref_n)} has no superseded-by: #{record.number}",
-                      gep_number: record.number
-                    }
-                  ]
-              else
-                acc
-              end
-            end
+            check_superseded_by(acc, record, number_map[ref_n], ref_n)
           end)
 
         # extended-by → see-also or requires on target
         acc =
           (record.extended_by || [])
           |> Enum.reduce(acc, fn ref_n, acc ->
-            ref = number_map[ref_n]
-
-            if ref == nil do
-              acc
-            else
-              back_refs = (ref.see_also || []) ++ (ref.requires || []) ++ (ref.extended_by || [])
-
-              if record.number not in back_refs do
-                acc ++
-                  [
-                    %{
-                      severity: :error,
-                      label: "Bidirectional links",
-                      detail:
-                        "GEP-#{gep_label(record.number)} extended-by GEP-#{gep_label(ref_n)} but GEP-#{gep_label(ref_n)} does not reference back",
-                      gep_number: record.number
-                    }
-                  ]
-              else
-                acc
-              end
-            end
+            check_back_ref(acc, record, number_map[ref_n], ref_n)
           end)
 
         acc
@@ -551,6 +511,46 @@ defmodule Gep.Validator do
     end
   end
 
+  defp check_superseded_by(acc, _record, nil, _ref_n), do: acc
+
+  defp check_superseded_by(acc, record, ref, ref_n) do
+    if ref.superseded_by == record.number do
+      acc
+    else
+      acc ++
+        [
+          %{
+            severity: :error,
+            label: "Bidirectional links",
+            detail:
+              "GEP-#{gep_label(record.number)} supersedes GEP-#{gep_label(ref_n)} but GEP-#{gep_label(ref_n)} has no superseded-by: #{record.number}",
+            gep_number: record.number
+          }
+        ]
+    end
+  end
+
+  defp check_back_ref(acc, _record, nil, _ref_n), do: acc
+
+  defp check_back_ref(acc, record, ref, ref_n) do
+    back_refs = (ref.see_also || []) ++ (ref.requires || []) ++ (ref.extended_by || [])
+
+    if record.number in back_refs do
+      acc
+    else
+      acc ++
+        [
+          %{
+            severity: :error,
+            label: "Bidirectional links",
+            detail:
+              "GEP-#{gep_label(record.number)} extended-by GEP-#{gep_label(ref_n)} but GEP-#{gep_label(ref_n)} does not reference back",
+            gep_number: record.number
+          }
+        ]
+    end
+  end
+
   defp check_cross_references(records, number_map) do
     all_refs =
       records
@@ -586,15 +586,7 @@ defmodule Gep.Validator do
   end
 
   defp check_readme_index(records, readme_path) do
-    unless File.exists?(readme_path) do
-      [
-        %{
-          severity: :error,
-          label: "README index",
-          detail: "README.md not found at #{readme_path}"
-        }
-      ]
-    else
+    if File.exists?(readme_path) do
       content = File.read!(readme_path)
 
       index_entries = parse_readme_index(content)
@@ -673,6 +665,14 @@ defmodule Gep.Validator do
         [] -> [%{severity: :pass, label: "README index", detail: "All entries valid"}]
         _ -> errors
       end
+    else
+      [
+        %{
+          severity: :error,
+          label: "README index",
+          detail: "README.md not found at #{readme_path}"
+        }
+      ]
     end
   end
 

test/gep/validator_test.exs

@@ -470,10 +470,9 @@ defmodule Gep.ValidatorTest do
 
   defp build_frontmatter(gep) do
     history_yaml =
-      Enum.map(gep.history, fn h ->
+      Enum.map_join(gep.history, "\n", fn h ->
         "  - date: #{h["date"]}\n    status: #{h["status"]}\n    note: #{h["note"]}"
       end)
-      |> Enum.join("\n")
 
     optional_fields =
       [
@@ -484,17 +483,16 @@ defmodule Gep.ValidatorTest do
         {"see-also", gep.see_also}
       ]
       |> Enum.reject(fn {_k, v} -> v == nil end)
-      |> Enum.map(fn {k, v} ->
+      |> Enum.map_join("\n", fn {k, v} ->
         yaml_val =
           if is_list(v) do
-            "[" <> Enum.join(Enum.map(v, &to_string/1), ", ") <> "]"
+            "[" <> Enum.map_join(v, ", ", &to_string/1) <> "]"
           else
             to_string(v)
           end
 
         "#{k}: #{yaml_val}"
       end)
-      |> Enum.join("\n")
 
     optional_block =
       if optional_fields != "" do