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

docs/Architecture.md

@@ -543,8 +543,8 @@ event, work item, cause); every verdict, escalation and external effect is logge
 - **MCP tools are the one deliberate exception:** fixed code against an API, with
   the model choosing only arguments. They still pass through P3, P4 and L4, and their
   output is redacted (L12).
-- **Tests use SSH too,** against a throwaway `sshd` container, so the tested path is
-  the production path.
+- **Tests use SSH too,** against a throwaway `sshd`, so the tested path is the
+  production path.
 
 ---