feat(proxy): report blocked packages on stop, exit 1 if any were blocked

pmg proxy stop now waits for the proxy process to exit, reads the final
blocked count written to the state file on shutdown, and exits non-zero
when one or more packages were blocked during the session.

This gives CI a clear failure signal from the proxy itself, separate
from the package manager's own exit code.

errcodes/codes.go

@@ -24,6 +24,9 @@ const (
 	CertTrustStore      = "CertTrustStore"
 	UnsupportedPlatform = "UnsupportedPlatform"
 
+	// Proxy error codes.
+	ProxyPackagesBlocked = "ProxyPackagesBlocked"
+
 	// Unknown mirrors the default code that dry/usefulerror returns for errors
 	// created without an explicit code, so unset and explicitly-unknown errors
 	// classify identically (e.g. the bug-report hint in ui.ErrorExit).