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>

README.md

@@ -1055,6 +1055,14 @@ go test ./...        # includes offline installer/update tests
 go build ./...       # build all packages
 ```
 
+End-to-end tests run against real services in a local Docker environment — two NetBox instances, SSH targets on separate networks, a jump host, an IPv6 host, tmux, zellij, and bash/zsh/fish. See [testenv/README.md](testenv/README.md). It is not part of CI/CD.
+
+```sh
+testenv/up.sh        # build and start (first start ~6 min)
+testenv/test.sh      # 141 checks: connect, strategies, instances, shortcuts, rules, sftp/scp, tunnels, notes, shells, TUI, configurator
+testenv/down.sh
+```
+
 The test suite covers the cache, NetBox client (via `httptest`), IP resolver chain, the argument parsers for ssh, sftp, and scp, SSH options and rules, port forwards, config loading, the setup wizard, shortcut normalization and rewrites, completion, the TUI keys, and shell hook install/uninstall. Wrapper tests run the real command (`sftp`, `scp`, `tunnel` with `--dry-run`) in a subprocess against a pre-filled cache, so no NetBox or ssh server is needed.
 
 Project layout: