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

conftest.py

@@ -0,0 +1,18 @@
+"""Fixtures shared by every test tree."""
+
+from collections.abc import Iterator
+from pathlib import Path
+
+import pytest
+
+from nisse_dev.execution import SSHHost
+from nisse_dev.testing import local_sshd
+
+
+@pytest.fixture(scope="session")
+def ssh_host(tmp_path_factory: pytest.TempPathFactory) -> Iterator[SSHHost]:
+    """A throwaway sshd on this machine, standing in for the build VM (L13).
+    It runs as the current user, so remote paths are this machine's paths and
+    tests can inspect what a command did."""
+    with local_sshd(Path(tmp_path_factory.mktemp("sshd"))) as host:
+        yield host