Fix/rails patterns followups

Independent local Codex review PASS at a6a6419402, no P0/P1. Independent 181 checks and strict skill validation pass. CI run 34667546951 passed, all 45 current checks green. Existing Rails skill mapping and documentation corrections only. Rollback: revert this squash commit.

config/project-stack-mappings.json

@@ -359,6 +359,7 @@
       ],
       "rules": ["common"],
       "skills": [
+        "rails-patterns",
         "tdd-workflow",
         "verification-loop"
       ],