test(installer): add OpenSSH distro harness

docs/implementation-roadmap.md

@@ -19,6 +19,17 @@ Worker repository:
   `main`;
 - document immutable SHA and release tags as production defaults.
 
+Source-install foundations landed:
+
+- [x] reusable Docker/OpenSSH harness and distro fixtures for Alpine, Ubuntu,
+      and Arch (`internal/installer/harness`) with real SSH readiness and
+      reliable teardown, gated behind `RSMON_TEST_DOCKER` (`make test-ssh`);
+- [x] pure distro/toolchain/source-install planning (`internal/sshinstall`):
+      os-release detection, package-manager/init resolution, pinned Go 1.26
+      toolchain with published SHA-256, and a plan the executor will run;
+- [ ] remote package install, Go download, clone, and build execution through
+      the SSH transport (source-install work package 3);
+
 Gate: a push publishes `sha-<12>` and `latest` manifests for both platforms,
 and a container remains healthy when the control plane is unavailable.