feat(remote): direct SSH targets without a gateway or scheduler

[ligon]

What

Adds a remote.host target so an ordinary Linux box can be a SuCoder target — no gateway hop, no DTN, no Slurm. Execution, Git transport, tunnels, and attach all ride the configured host or ~/.ssh/config alias, including after a connection expires.

targets:
  workstation:
    host: workstation.example.org  # or an existing ~/.ssh/config alias
    remote_user: ligon
    mirror_root: ~/mirrors

Notable design points

• Connection identity is resolved, not assumed. ssh -G evaluates the destination before a ControlMaster socket is chosen, so editing an alias or an Include file can't reuse a master still connected to the previous account or host. Resolution failure fails closed rather than falling back to a stale socket, and is deliberately uncached since SSH config can change between commands.
• remote_user wins over any User option, regardless of capitalization.
• host is mutually exclusive with gateway, transfer_host, slurm, and the BRC-specific cert_file; validation also rejects empty, whitespace-bearing, and --prefixed values (option injection).
• --node is refused consistently by collaborate, attach, and tunnel forward. A direct target has one host and no scheduler; _direct_control clears compute_node on connect, so a pin would silently vanish rather than take effect.
• release remains Slurm-only, and tunnel up warms the single connection without writing cluster aliases to ~/.ssh/config.

Testing

791 tests pass (763 pre-existing + 28 new). New coverage: config validation and mutual exclusion, socket identity separation by user/port/alias-resolution, Git transport option preservation, the full set of tunnel subcommands, and the --node refusal. The --node test is mutation-verified — it fails without the guard.

Reviewer notes — please read

• detect_changes reports CRITICAL risk: 34 symbols across 8 files, 44 affected execution flows (Collaborate, Attach, Tunnel_down, Push, Agents_clone). This is driven by breadth, not a detected defect — the feature necessarily touches every tunnel entry point. Every direct-target branch is gated on remote.host, which is None for all cluster targets. The one genuinely shared edit is executor.py's if self.control_socket_path or not self.gateway:; not self.gateway is always False for cluster targets because the parser requires a non-empty gateway.
• Not smoke-tested against a real SSH host. All coverage is mocked. Given the PR fix: harden startup and Slurm timer supervision #13 lesson — "the suite passed while the thing it checked was broken" — a manual sucoder -T <host> collaborate against a real box is worth doing before merge.
• ssh -G runs once per ssh_control_kwargs() call (local-only, 10s timeout). The fail-closed reasoning justifies it, but it is per-invocation rather than per-session.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AGhbys9tXLm9kDHGhxqLN6