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
Errors produced by Collectors.Host were keyword tuples {:tag, msg}, which
Jason cannot encode — metric push crashed the channel. Convert them to
plain maps with :tag and :message fields.
Reporter.handle_info/2 returned {:ok, socket}, which Slipstream rejects
(GenServer-style {:noreply, socket} is the only valid return for that
callback, unlike handle_connect/handle_join/handle_disconnect).
