test: add a local Docker test environment for all features

testenv/ starts two real NetBox 4.7 instances (v2 and v1 token), SSH
targets on public, mgmt, and prod networks, a jump host, an IPv6-only
host, a static host, and a client with ssh, sftp, scp, tmux, zellij,
bash, zsh, and fish. A seed script fills NetBox through the REST API.

141 end-to-end checks cover cache and refresh filters, connecting,
all resolver strategies, multiple instances, shortcuts and rewrites,
SSH options and rules, static hosts, sftp/scp incl. IPv6 and jump
hosts, tunnels, local notes and NetBox comments, completion and shell
hooks, the TUI (driven through tmux), the configurator, and uninstall.

Local only: testenv/up.sh, test.sh, shell.sh, down.sh. Not part of
CI/CD.

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

testenv/tests/run.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# Runs every tests/NN-*.sh (or those matching $1) in a fresh netssh home and
+# prints a summary. Exit code 1 if any check failed.
+set -u
+export RESULTS=/tmp/netssh-results
+: >"$RESULTS"
+filter=${1:-}
+for file in /tests/[0-9][0-9]-*.sh; do
+  name=$(basename "$file" .sh)
+  [[ -n $filter && $name != *"$filter"* ]] && continue
+  echo "== $name"
+  (
+    export CURRENT=$name
+    . /tests/lib.sh
+    new_home
+    . "$file"
+  )
+done
+passed=$(grep -c '^ok ' "$RESULTS" || true)
+failed=$(grep -c '^FAIL ' "$RESULTS" || true)
+echo
+echo "passed: $passed, failed: $failed"
+if [ "$failed" -gt 0 ]; then
+  grep '^FAIL ' "$RESULTS" | sed 's/^/  /'
+  exit 1
+fi