Keep credentials out of reprs; count only live ssh-agents in tests

A failing CI test printed a test key through SSHKey's default repr. Push
credentials, AgentConfig's tokens and MatrixConfig's token are now
excluded from their reprs, so no failure or log line can print them. In
CI's container killed ssh-agents linger as zombies, so the tests count
only live ones.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/tests/test_config.py

@@ -360,3 +360,10 @@ def test_a_push_token_needs_its_user_and_excludes_a_key() -> None:
         build(gitea__push_user="pusher")
     with pytest.raises(ConfigError, match="not both"):
         build(gitea__push_user="p", gitea__push_token="t", gitea__push_key="~/k")
+
+
+def test_no_credential_appears_in_the_config_repr() -> None:
+    config = build(gitea__push_user="pusher", gitea__push_token="${TEST_GITEA_TOKEN}")
+    text = repr(config)
+    for secret in SECRETS.values():
+        assert secret not in text