docs: update $ARGUMENTS syntax for v2.1.19 breaking change + evaluation

Updated all documentation and examples to reflect Claude Code v2.1.19
breaking change: $ARGUMENTS.0 → $ARGUMENTS[0] (bracket syntax).

Changes:
- guide/ultimate-guide.md: 7 occurrences updated to bracket/shorthand syntax
- guide/cheatsheet.md: Command template updated
- Added migration note in § 6.2 Variable Interpolation
- Created migration scripts: migrate-arguments-syntax.{sh,ps1}
  • Automated detection + conversion with backups
  • Dry-run mode, cross-platform (macOS/Linux/Windows)
- Added formal evaluation: eval-claude-code-releases-jan2026.md
  • Score: 5/5 (Critical - Integrate Immediately)
  • Covers releases 2.1.0 to 2.1.19 (January 2026)
  • Technical accuracy verified against GitHub CHANGELOG

Files:
- guide/ultimate-guide.md (+23 lines, 7 occurrences fixed)
- guide/cheatsheet.md (+1 line)
- examples/scripts/migrate-arguments-syntax.sh (+152 lines)
- examples/scripts/migrate-arguments-syntax.ps1 (+143 lines)
- docs/resource-evaluations/eval-claude-code-releases-jan2026.md (+273 lines)
- CHANGELOG.md (+12 lines, Unreleased section)

Total: +605 lines

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

examples/scripts/migrate-arguments-syntax.ps1

@@ -0,0 +1,143 @@
+# Migration Script: $ARGUMENTS syntax (v2.1.19 breaking change)
+#
+# Purpose: Update custom commands from old dot notation to new bracket syntax
+# Breaking Change: $ARGUMENTS.0 → $ARGUMENTS[0] (introduced in Claude Code v2.1.19)
+#
+# Usage:
+#   .\migrate-arguments-syntax.ps1           # Preview changes
+#   .\migrate-arguments-syntax.ps1 -Apply    # Apply changes
+#
+# Safety: Creates backups before modifying files
+
+param(
+    [switch]$Apply
+)
+
+# Configuration
+$BackupDir = Join-Path $env:USERPROFILE ".claude\backups\arguments-migration-$(Get-Date -Format 'yyyyMMdd-HHmmss')"
+
+# Directories to scan
+$ScanDirs = @(
+    (Join-Path $env:USERPROFILE ".claude\commands"),
+    (Join-Path $env:USERPROFILE ".claude\skills"),
+    ".claude\commands",
+    ".claude\skills"
+)
+
+Write-Host "╔═══════════════════════════════════════════════════════════╗" -ForegroundColor Blue
+Write-Host "║  Claude Code v2.1.19 - `$ARGUMENTS Syntax Migration      ║" -ForegroundColor Blue
+Write-Host "╚═══════════════════════════════════════════════════════════╝" -ForegroundColor Blue
+Write-Host ""
+Write-Host "Breaking Change: " -NoNewline -ForegroundColor Yellow
+Write-Host "`$ARGUMENTS.N → `$ARGUMENTS[N]"
+Write-Host ""
+
+# Check if any scan directories exist
+$foundDirs = $false
+foreach ($dir in $ScanDirs) {
+    if (Test-Path $dir) {
+        $foundDirs = $true
+        break
+    }
+}
+
+if (-not $foundDirs) {
+    Write-Host "✓ No custom commands/skills directories found" -ForegroundColor Green
+    Write-Host "  Nothing to migrate."
+    exit 0
+}
+
+# Find files with old syntax
+Write-Host "Scanning for files with old `$ARGUMENTS.N syntax..."
+Write-Host ""
+
+$affectedFiles = @()
+foreach ($dir in $ScanDirs) {
+    if (-not (Test-Path $dir)) {
+        continue
+    }
+
+    # Find .md files with $ARGUMENTS.N pattern
+    Get-ChildItem -Path $dir -Filter "*.md" -Recurse -ErrorAction SilentlyContinue | ForEach-Object {
+        $content = Get-Content $_.FullName -Raw
+        if ($content -match '\$ARGUMENTS\.[0-9]') {
+            $affectedFiles += $_.FullName
+        }
+    }
+}
+
+# Report findings
+if ($affectedFiles.Count -eq 0) {
+    Write-Host "✓ No files need migration" -ForegroundColor Green
+    Write-Host "  All custom commands already use the new syntax."
+    exit 0
+}
+
+Write-Host "Found $($affectedFiles.Count) file(s) with old syntax:" -ForegroundColor Yellow
+Write-Host ""
+
+# Preview changes
+foreach ($file in $affectedFiles) {
+    Write-Host "📄 $file" -ForegroundColor Blue
+
+    # Show occurrences
+    $lineNum = 0
+    Get-Content $file | ForEach-Object {
+        $lineNum++
+        if ($_ -match '\$ARGUMENTS\.[0-9]') {
+            Write-Host "   Line ${lineNum}: $_" -ForegroundColor Yellow
+        }
+    }
+    Write-Host ""
+}
+
+# Apply changes if requested
+if ($Apply) {
+    Write-Host "Creating backups in: $BackupDir" -ForegroundColor Yellow
+    New-Item -ItemType Directory -Path $BackupDir -Force | Out-Null
+
+    foreach ($file in $affectedFiles) {
+        # Create backup
+        $fileName = Split-Path $file -Leaf
+        $backupPath = Join-Path $BackupDir $fileName
+        Copy-Item $file $backupPath
+        Write-Host "✓ Backed up: $fileName" -ForegroundColor Green
+
+        # Apply migration
+        $content = Get-Content $file -Raw
+        $newContent = $content -replace '\$ARGUMENTS\.([0-9])', '$ARGUMENTS[$1]'
+        Set-Content -Path $file -Value $newContent -NoNewline
+
+        Write-Host "✓ Migrated: $file" -ForegroundColor Green
+    }
+
+    Write-Host ""
+    Write-Host "╔═══════════════════════════════════════════════════════════╗" -ForegroundColor Green
+    Write-Host "║  ✓ Migration Complete                                    ║" -ForegroundColor Green
+    Write-Host "╚═══════════════════════════════════════════════════════════╝" -ForegroundColor Green
+    Write-Host ""
+    Write-Host "Backups saved to: $BackupDir"
+    Write-Host ""
+    Write-Host "Changes applied:"
+    Write-Host "  • `$ARGUMENTS.0 → `$ARGUMENTS[0]"
+    Write-Host "  • `$ARGUMENTS.1 → `$ARGUMENTS[1]"
+    Write-Host "  • etc."
+    Write-Host ""
+    Write-Host "You can also use shorthand: `$0, `$1, `$2, ..."
+
+} else {
+    Write-Host "═══════════════════════════════════════════════════════════" -ForegroundColor Yellow
+    Write-Host "DRY RUN MODE - No changes applied" -ForegroundColor Yellow
+    Write-Host "═══════════════════════════════════════════════════════════" -ForegroundColor Yellow
+    Write-Host ""
+    Write-Host "To apply these changes, run:"
+    Write-Host "  .\migrate-arguments-syntax.ps1 -Apply" -ForegroundColor Green
+    Write-Host ""
+    Write-Host "This will:"
+    Write-Host "  1. Create backups in %USERPROFILE%\.claude\backups\"
+    Write-Host "  2. Update all files to new bracket syntax"
+    Write-Host "  3. Preserve original files in backup directory"
+}
+
+Write-Host ""
+Write-Host "Documentation: https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md#2119"