Run every command on the build VM over SSH (M1.3)

execution.SSHHost.execute is now the only way the agent runs anything
(L13). Commands travel as a script on SSH's stdin, so the secrets handed
to a call are never on a command line, and every value handed out is
redacted from the output. Timeouts kill the remote process group, and
an unreachable host is reported as such, not as a failed command.

Workspace and GitRepo run on the build VM. The clone uses the
repository's SSH URL, so no credential is stored in it (#89); fetches
and pushes get the push key over stdin, in a short-lived ssh-agent,
and PRs are still opened from the agent's host with its API token.
The bench runs on the build host too.

Tests reach "the build VM" through a throwaway local sshd (nisse_dev
.testing); CI installs openssh-server. The live push test now pushes
with the sandbox's deploy key (NISSE_PUSH_KEY).

Fixes #89.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/tests/test_config.py

@@ -32,6 +32,7 @@ COMPLETE: dict[str, Any] = {
         "repo": "don/demo",
         "operators": ["don"],
     },
+    "build_host": {"address": "build.test", "key": "~/.ssh/nisse-dev"},
 }
 SECRETS = {
     "TEST_LLM_TOKEN": "sk-x",
@@ -223,6 +224,9 @@ gitea:
   token: ${TEST_GITEA_TOKEN}
   repo: don/demo
   operators: [don]
+build_host:
+  address: build.test
+  key: ~/.ssh/nisse-dev
 """
 
 
@@ -328,3 +332,22 @@ def test_context_tuning_is_read_and_checked() -> None:
     assert build().tuning.context.keep_recent_turns == 6
     with pytest.raises(ConfigError, match="safety_margin: must be less than 1"):
         build(tuning={"context": {"safety_margin": 1}})
+
+
+def test_the_build_host_is_required_and_defaults_to_root_on_22() -> None:
+    config = build()
+    assert (config.build_host.address, config.build_host.user) == ("build.test", "root")
+    assert config.build_host.port == 22
+    assert config.build_host.key.is_absolute()
+    with pytest.raises(ConfigError, match="build_host.address"):
+        build(build_host__address=None)
+
+
+def test_a_bad_build_host_port_is_reported() -> None:
+    with pytest.raises(ConfigError, match="build_host.port"):
+        build(build_host__port="ssh")
+
+
+def test_the_push_key_is_optional_and_a_path() -> None:
+    assert build().push_key is None
+    assert build(gitea__push_key="~/k").push_key == Path("~/k").expanduser()