chore(agent): log /proc reads, log diagnostics enable, comment trap_exit

Addresses final code review:
- Host collector's /proc reads now go through Diagnostics.log_read/3,
  appearing in commands.log formatted as `$ cat /proc/loadavg`
- configure/1 logs an info line on successful enable so the operator
  has a breadcrumb in the journal
- Writer.init/1 documents the deliberate trap_exit omission

agent/lib/proxmox_agent/collectors/host.ex

@@ -51,13 +51,13 @@ defmodule ProxmoxAgent.Collectors.Host do
   end
 
   defp read_loadavg(proc_dir) do
-    body = File.read!(Path.join(proc_dir, "loadavg"))
+    body = read_and_log(Path.join(proc_dir, "loadavg"))
     [l1, l5, l15 | _] = String.split(body, ~r/\s+/, trim: true)
     {to_float(l1), to_float(l5), to_float(l15)}
   end
 
   defp read_meminfo(proc_dir) do
-    body = File.read!(Path.join(proc_dir, "meminfo"))
+    body = read_and_log(Path.join(proc_dir, "meminfo"))
 
     parsed =
       body
@@ -76,11 +76,23 @@ defmodule ProxmoxAgent.Collectors.Host do
   end
 
   defp read_uptime(proc_dir) do
-    body = File.read!(Path.join(proc_dir, "uptime"))
+    body = read_and_log(Path.join(proc_dir, "uptime"))
     [secs | _] = String.split(body, " ", trim: true)
     secs |> to_float() |> trunc()
   end
 
+  defp read_and_log(path) do
+    start = System.monotonic_time(:microsecond)
+    result = File.read(path)
+    duration_us = System.monotonic_time(:microsecond) - start
+    ProxmoxAgent.Diagnostics.log_read(path, result, duration_us)
+
+    case result do
+      {:ok, body} -> body
+      {:error, reason} -> raise File.Error, reason: reason, action: "read file", path: to_string(path)
+    end
+  end
+
   defp kb_to_bytes(nil), do: nil
 
   defp kb_to_bytes(str) do