feat: Adds React 18 and 19 migration plugin (#1339)

- Adds React 18 and 19 migration orchestration plugins
- Introduces comprehensive upgrade toolkits for migrating legacy React 16/17 and 18 codebases to React 18.3.1 and 19, respectively. Each plugin bundles specialized agents and skills for exhaustive audit, dependency management, class/component API migration, test suite transformation, and batching regression fixes.
- The React 18 toolkit targets class-component-heavy apps, ensures safe lifecycle and context transitions, resolves dependency blockers, and fully automates test migrations including Enzyme removal. The React 19 toolkit addresses breaking changes such as removal of legacy APIs, defaultProps on function components, and forwardRef, while enforcing a gated, memory-resumable migration pipeline.
- Both plugins update documentation, plugin registries, and skill references to support reliable, repeatable enterprise-scale React migrations.

skills/react-audit-grep-patterns/references/dep-scans.md

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+# Dependency Scans - Both Auditors
+
+Scans for dependency compatibility and peer conflicts. Run during both R18 and R19 audits.
+
+---
+
+## Current Versions
+
+```bash
+# All react-related package versions in one shot
+cat package.json | python3 -c "
+import sys, json
+d = json.load(sys.stdin)
+deps = {**d.get('dependencies',{}), **d.get('devDependencies',{})}
+keys = ['react', 'react-dom', 'react-router', 'react-router-dom',
+        '@testing-library/react', '@testing-library/jest-dom',
+        '@testing-library/user-event', '@apollo/client', 'graphql',
+        '@emotion/react', '@emotion/styled', 'jest', 'enzyme',
+        'react-redux', '@reduxjs/toolkit', 'prop-types']
+for k in keys:
+    if k in deps:
+        print(f'{k}: {deps[k]}')
+" 2>/dev/null
+```
+
+---
+
+## Peer Dependency Conflicts
+
+```bash
+# All peer dep warnings (must be 0 before migration completes)
+npm ls 2>&1 | grep -E "WARN|ERR|peer|invalid|unmet"
+
+# Count of peer errors
+npm ls 2>&1 | grep -E "WARN|ERR|peer|invalid|unmet" | wc -l
+
+# Specific package peer dep requirements
+npm info @testing-library/react peerDependencies 2>/dev/null
+npm info @apollo/client peerDependencies 2>/dev/null
+npm info @emotion/react peerDependencies 2>/dev/null
+npm info react-router-dom peerDependencies 2>/dev/null
+```
+
+---
+
+## Enzyme Detection (R18 Blocker)
+
+```bash
+# In package.json
+cat package.json | python3 -c "
+import sys, json
+d = json.load(sys.stdin)
+deps = {**d.get('dependencies',{}), **d.get('devDependencies',{})}
+enzyme = {k: v for k, v in deps.items() if 'enzyme' in k.lower()}
+if enzyme:
+    print('BLOCKER - Enzyme found:', enzyme)
+else:
+    print('No Enzyme - OK')
+" 2>/dev/null
+
+# Enzyme adapter files
+find . -name "enzyme-adapter*" -not -path "*/node_modules/*" 2>/dev/null
+```
+
+---
+
+## React Router Version Check
+
+```bash
+ROUTER=$(node -e "console.log(require('./node_modules/react-router-dom/package.json').version)" 2>/dev/null)
+echo "react-router-dom version: $ROUTER"
+
+# If v5 - flag for assessment
+if [[ $ROUTER == 5* ]]; then
+  echo "WARNING: react-router v5 found - requires scope assessment before upgrade"
+  echo "Run router migration scope scan:"
+  echo "  Routes: $(grep -rn "<Route\|<Switch\|<Redirect" src/ --include="*.js" --include="*.jsx" | grep -v "\.test\." | wc -l) hits"
+  echo "  useHistory: $(grep -rn "useHistory()" src/ --include="*.js" --include="*.jsx" | grep -v "\.test\." | wc -l) hits"
+fi
+```
+
+---
+
+## Lock File Consistency
+
+```bash
+# Check lockfile is in sync with package.json
+npm ls --depth=0 2>&1 | head -20
+
+# Check for duplicate react installs (can cause hooks errors)
+find node_modules -name "package.json" -path "*/react/package.json" 2>/dev/null \
+  | grep -v "node_modules/node_modules" \
+  | xargs grep '"version"' | sort -u
+```