feat: netssh multi — several hosts side by side in tmux or zellij

netssh multi opens one pane per host in a tmux window or zellij tab.
Hosts come from names, tag:TAG (NetBox and static hosts), cluster:NAME
(NetBox virtualization clusters; devices by default, --kind vm|all),
and groups from the config (@name, nested, with cycle detection).
Every pane runs netssh HOST, so lookup, rules, and jump hosts apply.

- tmux: a new window inside tmux, else a new session that is attached;
  tiled layout, pane borders with the host and a [SYNC] marker.
- zellij: a generated grid layout; a new tab inside zellij, else a new
  session.
- --sync broadcasts typing to all panes; netssh sync toggles it in the
  current tmux window or zellij tab.
- TUI: Space marks hosts, Ctrl+A marks all, Enter opens them together.
- Configurator: multiplexer, panes per window, groups.

NetBox SearchAll gains a cluster filter (name to cluster IDs per
instance), and cache entries keep the host's cluster for completion
and an offline fallback. Panes skip the 24-hour background refresh
(checked once by multi) and get HOME/XDG paths but no NETSSH_* values,
which would expose tokens in the process list.

testenv/tests/15-multi.sh checks real tmux and zellij sessions,
including a broadcast command arriving on all three cluster nodes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

testenv/tests/13-configure.sh

@@ -3,7 +3,7 @@
 config_only </dev/null
 rm -f "$XDG_CONFIG_HOME/netssh.yaml"
 #                          no NetBox, static hosts, add, name, address, user, port, tags, back, save
-out=$(python3 /tests/drive.py 2 4 1 legacy-box 10.231.0.50 admin "" legacy 5 8)
+out=$(python3 /tests/drive.py 2 4 1 legacy-box 10.231.0.50 admin "" legacy 5 9)
 [[ $out == *"Config saved"* ]] && ok "configure saves" || bad "configure saves" "$out"
 expect_out "config has the host" 'address: "10.231.0.50"' cat "$XDG_CONFIG_HOME/netssh.yaml"
 expect_eq "connect to the configured host" "admin" netssh legacy-box whoami