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/diagnostics.ex

@@ -22,6 +22,7 @@ defmodule ProxmoxAgent.Diagnostics do
     case File.mkdir_p(dir) do
       :ok ->
         Application.put_env(:agent, :dump_dir, dir)
+        Logger.info("diagnostics: enabled, writing to #{dir}")
         :ok
 
       {:error, reason} ->
@@ -46,6 +47,11 @@ defmodule ProxmoxAgent.Diagnostics do
     cast({:sample, kind, payload})
   end
 
+  @spec log_read(Path.t(), command_result(), non_neg_integer()) :: :ok
+  def log_read(path, result, duration_us) do
+    log_command("cat", [to_string(path)], result, duration_us)
+  end
+
   defp disable do
     Application.delete_env(:agent, :dump_dir)
     :ok