Merge pull request 'Run every command on the build VM over SSH (M1.3)' (#93) from ssh-execution into main

Reviewed-on: dongar/nisse#93
Reviewed-by: Don Garrett <dgarrett@acm.org>

docs/Development-Plan.md

@@ -282,8 +282,10 @@ if the implementation differs.
 - **D3 preflight:** a project's needs are read from its files (`Cargo.toml`,
   `pyproject.toml`, `package.json`) or a declaration in the repository.
 - **D4 publishing:** standard excludes are configured in each clone, and only what
-  the work produced is committed. A push hands the key over stdin to a
-  short-lived `ssh-agent` on the build VM, for that one push.
+  the work produced is committed. The push credential is handed to each push
+  only: a deploy key over stdin to a short-lived `ssh-agent` on the build VM, or
+  a push-only account's HTTPS token as that command's secrets, read by a
+  credential helper that names environment variables, never values.
 - **D7:** test services use ports 18000–18099, so opening access later is a
   firewall change. The inference host is behind LiteLLM, so it is not a second
   egress exception.