feat(data-platform): add schema diff detection hook (#228)

Implements PostToolUse hook to warn about potentially breaking schema changes:
- DROP COLUMN/TABLE/INDEX detection
- Column type changes (ALTER TYPE, MODIFY COLUMN)
- NOT NULL constraint additions
- RENAME operations
- ORM model field removals

Non-blocking - outputs warnings only.
Configurable via DATA_PLATFORM_SCHEMA_WARN env var.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/data-platform/hooks/hooks.json

@@ -5,6 +5,17 @@
         "type": "command",
         "command": "${CLAUDE_PLUGIN_ROOT}/hooks/startup-check.sh"
       }
+    ],
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/schema-diff-check.sh"
+          }
+        ]
+      }
     ]
   }
 }