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

tests/integration/nisse-test.yaml

@@ -7,11 +7,15 @@
 # the live tests that need it, rather than skipping them, so a broken
 # credential cannot hide as a skip.
 
-# Unused by the live tests, which work in pytest's temporary directories;
-# present because every agent config has them.
+# Unused by the live tests, which work in pytest's temporary directories on a
+# throwaway local sshd (the `ssh_host` fixture); present because every agent
+# config has them.
 state_db: /tmp/nisse-test/state.db
 workspace: /tmp/nisse-test/work
 project: nisse-dev-testing
+build_host:
+  address: 127.0.0.1
+  key: /tmp/nisse-test/unused-key
 
 llm:
   base_url: https://litellm.dongar.net