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

.env.example

@@ -11,6 +11,9 @@ NISSE_LLM_TOKEN=replace-me
 NISSE_MATRIX_TOKEN=replace-me
 # gitea.token: the Gitea service account's scoped token (B2)
 NISSE_GITEA_TOKEN=replace-me
+# The sandbox repository's deploy key (write access), for the live push test:
+# the private key itself, or here, since a .env line can't hold one, its path.
+NISSE_PUSH_KEY=/root/nisse-e2e/ssh/push_sandbox
 
 # Where the config file is, if not ./nisse-dev.yaml. Where this file is, if
 # not ./.env, is NISSE_ENV, which has to be set in the real environment.