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/src/nisse_dev/testing.py

@@ -0,0 +1,101 @@
+"""Test support: a throwaway sshd standing in for a tool VM (L13).
+
+Runs the system's sshd as the current user, on a free local port, with a
+host key and client key made for the occasion. Tests then reach "the build
+VM" over real SSH, so the code path under test is the production path.
+"""
+
+import getpass
+import os
+import socket
+import subprocess
+import time
+from collections.abc import Iterator
+from contextlib import contextmanager
+from pathlib import Path
+
+from nisse_dev.execution import SSHHost
+
+SSHD = "/usr/sbin/sshd"
+
+
+def _free_port() -> int:
+    with socket.socket() as sock:
+        sock.bind(("127.0.0.1", 0))
+        return int(sock.getsockname()[1])
+
+
+def _keygen(path: Path) -> None:
+    subprocess.run(
+        ["ssh-keygen", "-q", "-t", "ed25519", "-N", "", "-f", str(path)], check=True
+    )
+
+
+@contextmanager
+def local_sshd(root: Path) -> Iterator[SSHHost]:
+    """An SSHHost for a private sshd on 127.0.0.1, torn down afterwards."""
+    root.mkdir(parents=True, exist_ok=True)
+    host_key, client_key = root / "host_key", root / "client_key"
+    _keygen(host_key)
+    _keygen(client_key)
+    authorized = root / "authorized_keys"
+    authorized.write_text(client_key.with_suffix(".pub").read_text())
+    authorized.chmod(0o600)
+    port = _free_port()
+    config = root / "sshd_config"
+    config.write_text(
+        "\n".join(
+            [
+                f"Port {port}",
+                "ListenAddress 127.0.0.1",
+                f"HostKey {host_key}",
+                f"AuthorizedKeysFile {authorized}",
+                f"PidFile {root / 'sshd.pid'}",
+                "PasswordAuthentication no",
+                "KbdInteractiveAuthentication no",
+                "PubkeyAuthentication yes",
+                "PermitRootLogin prohibit-password",
+                "StrictModes no",
+                "UsePAM no",
+                "MaxSessions 50",
+                "MaxStartups 50",
+                # Stands in for a provisioned build VM: sessions see the test
+                # runner's PATH, so its Python and tools are "installed".
+                f"SetEnv PATH={os.environ.get('PATH', '/usr/bin:/bin')}",
+                "",
+            ]
+        )
+    )
+    process = subprocess.Popen(
+        [SSHD, "-D", "-e", "-f", str(config)],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.PIPE,
+    )
+    user = getpass.getuser()
+    host = SSHHost(
+        "127.0.0.1", key=client_key, state_dir=root / "client", user=user, port=port
+    )
+    try:
+        deadline = time.monotonic() + 10
+        while True:
+            if process.poll() is not None:
+                raise RuntimeError(
+                    "sshd exited: " + (process.stderr.read() or b"").decode()
+                    if process.stderr
+                    else "sshd exited"
+                )
+            try:
+                with socket.create_connection(("127.0.0.1", port), timeout=0.5):
+                    break
+            except OSError:
+                if time.monotonic() > deadline:
+                    raise RuntimeError("sshd did not start") from None
+                time.sleep(0.05)
+        yield host
+    finally:
+        host.close()
+        process.terminate()
+        try:
+            process.wait(timeout=5)
+        except subprocess.TimeoutExpired:
+            process.kill()