feat(proxy): add persistent proxy server with start/stop/env/status commands

Adds pmg proxy command group for running a long-lived MITM proxy that
intercepts all package manager traffic without requiring PMG shims or
wrappers. Targets CI/CD pipelines where env vars can be set globally.

- pmg proxy start: starts proxy with npm+pypi interceptors, writes
  state file (pid/addr/ca-cert-path), auto-blocks suspicious packages
- pmg proxy stop: sends SIGTERM to the running proxy
- pmg proxy env: emits HTTP_PROXY/HTTPS_PROXY/SSL_CERT_FILE etc. as
  shell exports, or writes directly to $GITHUB_ENV with --gha
- pmg proxy status: shows running/stopped status

GHA usage:
  pmg proxy start &
  pmg proxy env --gha   # populates env for all subsequent steps
  npm install           # intercepted automatically, no wrapper needed

Also adds .github/workflows/persistent-proxy-e2e.yml to validate the
persistent proxy mode end-to-end in CI.

cmd/proxy/proxy.go

@@ -0,0 +1,15 @@
+package proxy
+
+import "github.com/spf13/cobra"
+
+func NewProxyCommand() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "proxy",
+		Short: "Manage the persistent PMG proxy server",
+	}
+	cmd.AddCommand(newStartCommand())
+	cmd.AddCommand(newStopCommand())
+	cmd.AddCommand(newEnvCommand())
+	cmd.AddCommand(newStatusCommand())
+	return cmd
+}