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

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

agent/lib/proxmox_agent/diagnostics/writer.ex

@@ -19,6 +19,9 @@ defmodule ProxmoxAgent.Diagnostics.Writer do
   def init(opts) do
     dir = Keyword.fetch!(opts, :dir)
 
+    # No trap_exit: the Writer owns no linked processes other than the supervisor.
+    # If you add a linked port/task, restore Process.flag(:trap_exit, true) and
+    # add a matching handle_info({:EXIT, _, _}, state) clause.
     with {:ok, commands} <- open(Path.join(dir, "commands.log")),
          {:ok, samples} <- open(Path.join(dir, "samples.log")) do
       {:ok, %{dir: dir, commands: commands, samples: samples}}