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>

CHANGELOG.md

@@ -5,7 +5,9 @@
 - Fix the `interface_name` resolver strategy: it sent filter names that NetBox ignores and therefore returned the host's first IP address instead of the named interface's. It now uses NetBox's `interface` and `vminterface` filters; a missing interface falls through to the next strategy.
 - Fix `netssh sftp` and `netssh scp` with instance-qualified hosts (`prod/web01:/path`), which were taken for local paths.
 - Fix shell aliases of instance-qualified hosts ignoring anchored rewrite rules (`fsn_fsn1-pve1` instead of `fsn_fsnpve1`).
-- Add `testenv/`, a local Docker environment with two NetBox instances, SSH targets, a jump host, tmux, zellij, and bash/zsh/fish, and 141 end-to-end checks of all features (`testenv/up.sh`, `testenv/test.sh`). Not part of CI/CD.
+- Add `netssh multi` to open several hosts side by side in tmux or zellij: host lists, `tag:`, NetBox virtualization clusters (`cluster:`, devices by default, `--kind vm|all`), and groups from the config (`@name`). `--sync` sends typing to all panes; `netssh sync` toggles it later. In the TUI, `Space` marks hosts and `Enter` opens them together. Configurable in `netssh configure` (multiplexer, panes per window, groups).
+- Cache entries remember the NetBox cluster of a host, for `cluster:` completion and offline use.
+- Add `testenv/`, a local Docker environment with two NetBox instances, SSH targets, a jump host, tmux, zellij, and bash/zsh/fish, and 160 end-to-end checks of all features, including `netssh multi` with real tmux and zellij (`testenv/up.sh`, `testenv/test.sh`). Not part of CI/CD.
 
 ## v0.0.23