fix: enable git-over-ssh cloning for private repo installs

The manager image had git but not openssh-client, so ssh:// clones
failed outright ("cannot run ssh"). Install openssh-client, and mount
an SSH_DIR (key + known_hosts) into the container read-only so private
repos can actually authenticate — confirmed both gaps while deploying
against real private Zenvairo repos.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -28,3 +28,8 @@ MASTER_KEY=change-me-to-a-random-string-of-at-least-32-characters
 # Resource caps applied to every installed MCP container (same limit for all, no per-installation override in V1)
 CONTAINER_MEMORY_MB=512
 CONTAINER_CPUS=1
+
+# Absolute path to an SSH dir (key + known_hosts) with clone access to the repos you'll
+# install, mounted read-only into the manager container so `git clone ssh://...` works for
+# private repos. Required only if you install from private/SSH repo URLs.
+SSH_DIR=/root/.ssh