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

@@ -34,6 +34,7 @@ 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
@@ -146,6 +147,11 @@ run_bootstrap() {
   exec "$MOSAIC_HOME/bin/mosaic-bootstrap-repo" "$@"
 }
 
+run_upgrade() {
+  check_mosaic_home
+  exec "$MOSAIC_HOME/bin/mosaic-upgrade" "$@"
+}
+
 # Main router
 if [[ $# -eq 0 ]]; then
   usage
@@ -163,6 +169,7 @@ case "$command" in
   doctor)     run_doctor "$@" ;;
   sync)       run_sync "$@" ;;
   bootstrap)  run_bootstrap "$@" ;;
+  upgrade)    run_upgrade "$@" ;;
   help|-h|--help)  usage ;;
   version|-v|--version) echo "mosaic $VERSION" ;;
   *)