feat: update hbc via hb_install.sh instead of code patching

Server now sends a bare UPD command; client runs hb_install.sh to
reinstall from the package registry, then restarts. hb_install.sh
also copies itself alongside hbc on client installs.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

hbd/server/http.py

@@ -210,15 +210,11 @@ async def start(
             return err
         qa = request.rel_url.query
         uname = urllib.parse.unquote(qa.get("h", ""))
-        ucode = qa.get("c")
-        if not ucode or not uname:
-            return web.Response(status=400, text="need h= and c= arguments")
+        if not uname:
+            return web.Response(status=400, text="need h= argument")
         if uname != "All" and uname not in hbdclass.Host.hosts:
             return web.Response(status=400, text=f"h={uname} not found")
-        if uname != "All":
-            names = [uname]
-        else:
-            names = [n for n in hbdclass.Host.hosts]
+        names = [uname] if uname != "All" else list(hbdclass.Host.hosts)
         out = []
         for n in names:
             host = hbdclass.Host.hosts[n]
@@ -227,8 +223,7 @@ async def start(
                 continue
             op_err = None
             try:
-                r = {"csum": None, "code": ucode}
-                host.cmds.append(("UPD", r))
+                host.cmds.append(("UPD", {}))
             except Exception as e:
                 op_err = str(e)
             out.append(f"update started for {n}: {op_err if op_err else 'OK'}")