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/08-transfer.sh

@@ -0,0 +1,24 @@
+# sftp and scp: upload, download, jump host, IPv6, errors.
+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
+echo "payload $$" >"$HOME/up.txt"
+check "scp upload" netssh scp "$HOME/up.txt" fsn/fsn1-pve1.hoster.test:/tmp/up.txt
+expect_eq "uploaded content" "payload $$" netssh fsn/fsn1-pve1.hoster.test cat /tmp/up.txt
+check "scp download" netssh scp fsn/fsn1-pve1.hoster.test:/tmp/up.txt "$HOME/down.txt"
+expect_eq "downloaded content" "payload $$" cat "$HOME/down.txt"
+check "scp to a VM through the bastion" netssh scp "$HOME/up.txt" fsn/web1:/tmp/up.txt
+expect_eq "VM content" "payload $$" netssh fsn/web1 cat /tmp/up.txt
+check "scp to IPv6 host" netssh scp "$HOME/up.txt" fsn/v6-host:/tmp/v6.txt
+expect_eq "IPv6 content" "payload $$" netssh fsn/v6-host cat /tmp/v6.txt
+expect_out "sftp starts in the path" "Remote working directory: /tmp" bash -c 'echo pwd | netssh sftp -b - fsn/fsn1-pve2.hoster.test:/tmp'
+expect_out "sftp to IPv6 host" "v6.txt" bash -c 'echo "ls /tmp" | netssh sftp -b - fsn/v6-host'
+expect_out "sftp through the bastion" "up.txt" bash -c 'echo "ls /tmp" | netssh sftp -b - fsn/web1'
+expect_fail "two remote hosts" "not supported" netssh scp fsn/web1:/a fsn/web2:/b