feat: add mosaic upgrade command to clean stale per-project files

Detects and cleans up files that are now centralized:
- SOUL.md: removed (now global at ~/.config/mosaic/SOUL.md)
- CLAUDE.md: replaced with thin pointer to global AGENTS.md
- AGENTS.md: stale load-order sections stripped, project content preserved

Supports --dry-run, --all (scan ~/src/*), and per-project paths.
Creates .mosaic-bak backups before any modification.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/mosaic.ps1

@@ -31,6 +31,7 @@ Management:
   doctor [args...]       Audit runtime state and detect drift
   sync [args...]         Sync skills from canonical source
   bootstrap <path>       Bootstrap a repo with Mosaic standards
+  upgrade [path]         Clean up stale SOUL.md/CLAUDE.md in a project
 
 Options:
   -h, --help             Show this help
@@ -140,6 +141,11 @@ switch ($command) {
         Write-Host "[mosaic] NOTE: mosaic-bootstrap-repo requires bash. Use Git Bash or WSL." -ForegroundColor Yellow
         & (Join-Path $MosaicHome "bin\mosaic-bootstrap-repo") @remaining
     }
+    "upgrade" {
+        Assert-MosaicHome
+        Write-Host "[mosaic] NOTE: mosaic-upgrade requires bash. Use Git Bash or WSL." -ForegroundColor Yellow
+        & (Join-Path $MosaicHome "bin\mosaic-upgrade") @remaining
+    }
     { $_ -in "help", "-h", "--help" } { Show-Usage }
     { $_ -in "version", "-v", "--version" } { Write-Host "mosaic $Version" }
     default {