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/src/nisse_dev/config.py

@@ -233,12 +233,14 @@ class AgentConfig:
     workspace: Path
     project: str
     llm_base_url: str
-    llm_token: str
+    # Credentials never appear in a repr: a failing test or a log line
+    # would print them.
+    llm_token: str = field(repr=False)
     roles: RoleMap
     matrix: MatrixConfig
     operators: frozenset[str]
     gitea_url: str
-    gitea_token: str
+    gitea_token: str = field(repr=False)
     gitea_repo: str
     gitea_operators: frozenset[str]
     build_host: BuildHost
@@ -248,7 +250,7 @@ class AgentConfig:
     # (push_user, push_token), or the path to a deploy key (push_key).
     push_key: Path | None = None
     push_user: str | None = None
-    push_token: str | None = None
+    push_token: str | None = field(default=None, repr=False)
     max_output_tokens: int = DEFAULT_MAX_OUTPUT_TOKENS
     request_timeout_s: float = DEFAULT_REQUEST_TIMEOUT_S
     tuning: Tuning = field(default_factory=Tuning)