feat(installer): multi-instance install with full env resolution

Rework `rsmon-worker install` so one host can run several isolated
workers and so the installer consumes the full worker env-var set.

- main.go now loads .env before dispatching management commands, so
  install/deploy read the same environment as the runtime.
- New --name flag installs a co-located worker as rsmon-worker-<name>
  with its own binary (/usr/local/bin/rsmon-worker-<name>), config
  (/etc/rsmon-worker-<name>), data dir (/var/lib/rsmon-worker-<name>),
  and systemd unit. Named instances require an explicit WORKER_PORT.
- Configuration is resolved flags > --env-file > process env/.env
  (godotenv) > defaults; the resolved set is written as a stable,
  systemd-safe 0600 env file.
- WORKER_LOGIN/WORKER_PASSWORD default to a generated admin password
  (printed once) when both are unset; XOR is rejected.
- The generated unit is now hardened (After=docker.service, CAP_NET_RAW,
  ProtectSystem=full, ReadWritePaths=data dir) and parameterized by
  instance; the Docker unit is namespaced by instance too.
- install creates the data + config directories and prints a summary
  (unit, binary, env file, data dir, console URL, generated password).
- New flags: --name, --host, --port, --login, --password/--password-file.
- Tests: resolvePaths, validateInstanceName, resolveInstallEnv
  precedence/XOR/port-required, renderEnvFile, validateEnvValue, plus
  named-instance unit assertions. End-to-end verified by installing and
  removing a throwaway --name instance.
- docs/install.md documents the tool, config sources/precedence,
  single- and multi-instance flows, the exact actions performed, the
  generated unit, options, and uninstall.

README.md

@@ -77,6 +77,13 @@ Docker image references. The Harbor project is appended separately as `rsmon`.
 
 ## systemd
 
+The `install` subcommand turns a built binary into an enabled systemd service:
+it writes a hardened unit, a mode-0600 env file, the data directory, then
+enables and starts the worker. It reads configuration from flags, `--env-file`,
+a `.env` in the working directory, or the process environment (in that order),
+and supports several co-located workers per host via `--name`. The full
+reference is in [`docs/install.md`](docs/install.md).
+
 Install host dependencies first. On Debian or Ubuntu:
 
 ```bash