feat: Packer build orchestration (SAND-WP-0012)

Add vm-packer build mode, profile.vm-packer-build, State Hub progress
notes during long provision, docs/runbook, and build mode tests.

src/sandboxer/core/manager.py

@@ -131,6 +131,16 @@ class SandboxManager:
         try:
             secret_bundle = resolve_setup_secrets(profile)
             provision_inputs = dict(request.inputs)
+            build_mode = (
+                provision_inputs.get("mode") == "build"
+                or profile.id == "profile.vm-packer-build"
+            )
+            if build_mode:
+                emit_lifecycle_event(
+                    status,
+                    summary=f"Packer build starting ({profile.id})",
+                    event_type="note",
+                )
             handle = backend.provision(profile, provision_inputs, resolved_host)
             if secret_bundle:
                 handle["_secret_refs"] = secret_bundle