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_publish.py

@@ -5,12 +5,22 @@ from typing import Any
 
 import pytest
 
+from nisse_dev.execution import SSHHost
 from nisse_dev.git import GitRepo
 from nisse_dev.gitea import GiteaError, PullRequest
 from nisse_dev.oplog import OpLog
 from nisse_dev.publish import OPEN_PR_INTENT, MilestonePublisher
 from nisse_dev.state import StateStore
 
+# The build VM for every test here: the session's throwaway sshd.
+HOST: SSHHost
+
+
+@pytest.fixture(autouse=True)
+def _build_host(ssh_host: SSHHost) -> None:
+    global HOST
+    HOST = ssh_host
+
 
 class FakeGitea:
     def __init__(self, *, fail: bool = False) -> None:
@@ -46,7 +56,7 @@ class FakeGit:
     def __init__(
         self, root: Path, *, changes: bool = True, committed: bool = False
     ) -> None:
-        self.repo = GitRepo(root)
+        self.repo = GitRepo(HOST, root)
         self.repo.init()
         self.repo.configure_identity("bot", "bot@example.test")
         (root / "README.md").write_text("sandbox")
@@ -67,7 +77,7 @@ class FakeGit:
         return self.repo.commits_since(ref)
 
     def remote_sha(
-        self, remote_url: str, branch: str, *, token: str | None = None
+        self, remote_url: str, branch: str, *, key: str | None = None
     ) -> str | None:
         return self.remote.get(branch)
 
@@ -80,7 +90,7 @@ class FakeGit:
     def commit_all(self, message: str) -> str | None:
         return self.repo.commit_all(message)
 
-    def push(self, remote_url: str, branch: str, *, token: str | None = None) -> None:
+    def push(self, remote_url: str, branch: str, *, key: str | None = None) -> None:
         self.pushed.append(branch)
         self.remote[branch] = self.repo.head_sha()
 
@@ -100,9 +110,8 @@ def build(store: StateStore, git: Any, client: Any) -> MilestonePublisher:
         client,
         "o/sandbox",
         git,
-        base_url="https://gitea.test",
-        account="bot",
-        token="tok",
+        remote_url="ssh://git@gitea.test/o/sandbox.git",
+        push_key="KEY",
     )