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/04-instances.sh

@@ -0,0 +1,20 @@
+# Two NetBox instances: qualified names, ambiguity, an unreachable instance.
+config <<'EOF'
+resolver:
+  strategies: [management_subnet, primary_ip]
+  management_subnets: [10.231.1.0/24]
+ssh:
+  default_user: ops
+  rules:
+    - match: {kind: vm, instance: fsn}
+      proxy_jump: ops@10.231.0.30
+EOF
+expect_fail "same name in both instances is ambiguous" "found 2 exact matches" netssh --dry-run web1
+expect_eq "fsn/web1 through the bastion" "web1" netssh fsn/web1 hostname
+expect_eq "lab/web1 directly" "lab-web1" netssh lab/web1 hostname
+expect_eq "lab uses its v1 token" "lab-db1" netssh lab/lab-db1 hostname
+
+# lab unreachable: qualified fsn targets still work, unqualified ones need every instance.
+sed -i 's#http://10.231.0.21:8080#http://10.231.0.21:9#' "$XDG_CONFIG_HOME/netssh.yaml"
+expect_eq "qualified target with another instance down" "fsn1-pve1" netssh fsn/fsn1-pve1.hoster.test hostname
+expect_fail "unqualified target needs every instance" "10.231.0.21:9" netssh --dry-run fsn1-pve1.hoster.test