feat(procmgr): supervise process-agent on Windows via dd-procmgr (split PR 4/4)

Wire dual-mode agent startup, fleet processes.d install for process-agent,
dd-procmgr-service LocalSystem credentials, and Windows E2E coverage. Completes
the process-agent migration off legacy SCM when procmgr owns supervision.

Author: jose-manuel-almaza

cmd/agent/subcommands/run/BUILD.bazel

@@ -290,6 +290,7 @@ go_library(
             "//comp/trace/etwtracer/fx",
             "//pkg/config/model",
             "//pkg/util/winutil",
+            "@org_golang_x_sys//windows/svc",
         ],
         "//conditions:default": [],
     }),
@@ -300,6 +301,7 @@ dd_agent_go_test(
     srcs = [
         "command_test.go",
         "command_windows_test.go",
+        "dependent_services_windows_test.go",
     ],
     embed = [":run"],
     deps = [
@@ -308,5 +310,12 @@ dd_agent_go_test(
         "//comp/core/pid/impl",
         "//pkg/util/fxutil",
         "@com_github_stretchr_testify//require",
-    ],
+    ] + select({
+        "@rules_go//go/platform:windows": [
+            "//pkg/config/mock",
+            "//pkg/config/model",
+            "@com_github_stretchr_testify//assert",
+        ],
+        "//conditions:default": [],
+    }),
 )