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/lifecycle/state_hub.py

@@ -59,6 +59,37 @@ def emit_lifecycle_event(
         return None
 
 
+def emit_progress_note(
+    summary: str,
+    *,
+    sandbox_id: str,
+    profile_id: str,
+    detail: dict[str, Any] | None = None,
+    author: str = "sandboxer",
+) -> dict[str, Any] | None:
+    """Emit a progress note during long-running provision (e.g. Packer build)."""
+    if os.environ.get("SANDBOXER_NO_STATE_HUB", "").lower() in ("1", "true", "yes"):
+        return None
+
+    payload = {
+        "event_type": "note",
+        "summary": summary,
+        "author": author,
+        "detail": {
+            "sandbox_id": sandbox_id,
+            "profile_id": profile_id,
+            **(detail or {}),
+        },
+    }
+
+    try:
+        response = httpx.post(f"{hub_url()}/progress/", json=payload, timeout=10.0)
+        response.raise_for_status()
+        return response.json()
+    except httpx.HTTPError:
+        return None
+
+
 def event_type_for_state(state: SandboxState) -> str:
     if state in (SandboxState.READY, SandboxState.DESTROYED, SandboxState.EXPIRED):
         return "milestone"