feat: add dynamic per-call SSH connections and secret redaction

- execute-command/upload/download accept host/port/username/password inline
  to open an ephemeral connection for a single call
- add redactSecret/redactSecrets to mask credentials in logs and errors
- logger log()/handleError() accept optional secrets[] for redaction
- add SSH_CONFIG_MISSING error code and --password-from-env CLI flag
- dynamic-mode startup: server runs with no static config

LICENSE

@@ -0,0 +1,15 @@
+ISC License
+
+Copyright (c) 2025 junki.cn
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.