docs(remote-updates): document shared venv, self-update argv mechanics, new symptom rows
- Install layout now shows shared /opt/decnet/venv/ (not per-slot .venv/)
and the persistent /opt/decnet/.env.local convention.
- 'What the updater does' reflects: full-dep bootstrap on first venv use
then --no-deps; /proc-scan fallback when agent.pid is absent.
- --include-self section spells out the argv reconstruction: why sys.argv
is wrong inside the app process and which env vars the new argv is
rebuilt from.
- Symptom table picks up three rows from the recent fixes (--ssl-keyfile
crash, missing typer after first self-update, old agent surviving the
restart when it was started manually).
