Fix SSH auth: respect ~/.ssh/config instead of defaulting to root

CoulombCore (92.205.130.254) authenticates as tegwick via id_ops in
ssh config, not root. Omit SANDBOXER_SSH_USER to let OpenSSH apply config;
set SANDBOXER_SSH_USER only to override.

src/sandboxer/extensions/compose_ssh.py

@@ -18,7 +18,7 @@ class ComposeSSHExtension:
     def __init__(self, config: dict[str, Any] | None = None) -> None:
         cfg = config or {}
         self.base_dir: str = cfg.get("base_dir", "/tmp/sandboxer")
-        self.ssh_user: str = cfg.get("ssh_user", "root")
+        self.ssh_user: str | None = cfg.get("ssh_user")
         self.compose_timeout_s: int = int(cfg.get("compose_timeout_s", 180))
 
     def provision(
@@ -33,7 +33,7 @@ class ComposeSSHExtension:
 
         sandbox_id = inputs.get("sandbox_id") or str(uuid.uuid4())[:8]
         remote_dir = f"{self.base_dir}/{sandbox_id}"
-        ssh = SSHConfig.from_env(host, user=self.ssh_user)
+        ssh = SSHConfig.from_env(host, user=self.ssh_user or None)
 
         rc, out = ssh.run(f"mkdir -p {remote_dir}")
         if rc != 0:
@@ -64,7 +64,8 @@ class ComposeSSHExtension:
 
     def wait_ready(self, handle: dict[str, str]) -> dict[str, str]:
         """Confirm compose services are running (no HTTP health polling)."""
-        ssh = SSHConfig.from_env(handle["host"], user=handle.get("ssh_user", self.ssh_user))
+        ssh_user = handle.get("ssh_user") or self.ssh_user or None
+        ssh = SSHConfig.from_env(handle["host"], user=ssh_user)
         project = handle["compose_project"]
         remote_dir = handle["remote_dir"]
         compose_file = handle["compose_file"]
@@ -83,7 +84,8 @@ class ComposeSSHExtension:
         }
 
     def teardown(self, handle: dict[str, str]) -> dict[str, str]:
-        ssh = SSHConfig.from_env(handle["host"], user=handle.get("ssh_user", self.ssh_user))
+        ssh_user = handle.get("ssh_user") or self.ssh_user or None
+        ssh = SSHConfig.from_env(handle["host"], user=ssh_user)
         project = handle.get("compose_project")
         remote_dir = handle.get("remote_dir")
         compose_file = handle.get("compose_file")

src/sandboxer/extensions/ssh.py

@@ -11,19 +11,27 @@ from pathlib import Path
 @dataclass
 class SSHConfig:
     host: str
-    user: str = "root"
+    user: str | None = None
     key: str | None = None
     connect_timeout: int = 15
 
+    @property
+    def destination(self) -> str:
+        """SSH destination; omit user when unset so ~/.ssh/config applies."""
+        if self.user:
+            return f"{self.user}@{self.host}"
+        return self.host
+
     @property
     def target(self) -> str:
-        return f"{self.user}@{self.host}"
+        return self.destination
 
     @classmethod
     def from_env(cls, host: str, *, user: str | None = None, key: str | None = None) -> SSHConfig:
+        env_user = os.environ.get("SANDBOXER_SSH_USER")
         return cls(
             host=host,
-            user=user or os.environ.get("SANDBOXER_SSH_USER", "root"),
+            user=user if user is not None else (env_user or None),
             key=key or os.environ.get("SANDBOXER_SSH_KEY"),
         )
 
@@ -39,7 +47,7 @@ class SSHConfig:
         ]
         if self.key:
             args += ["-i", self.key]
-        args.append(self.target)
+        args.append(self.destination)
         return args
 
     def run(self, cmd: str, *, timeout: int = 60) -> tuple[int, str]: