toolsrv: add Dial() with Addr type for unified construction

Addr{Network, Host, CWD} describes how to reach a tool server.
Dial dispatches to local (in-process) or ssh (remote) based on Network.
TODO comment describes future path: local via unix socket.

toolsrv/dial.go

@@ -0,0 +1,55 @@
+package toolsrv
+
+import (
+	"context"
+	"fmt"
+)
+
+// Addr describes how to reach a tool server.
+type Addr struct {
+	// Network is the transport: "local", "ssh", or (future) "unix".
+	Network string
+	// Host is the SSH target (user@host or user@host:port). Empty for local.
+	Host string
+	// CWD is the working directory on the target.
+	CWD string
+}
+
+// LocalAddr returns an Addr for an in-process tool server.
+func LocalAddr(cwd string) Addr {
+	return Addr{Network: "local", CWD: cwd}
+}
+
+// SSHAddr returns an Addr for a remote tool server over SSH.
+func SSHAddr(host, cwd string) Addr {
+	return Addr{Network: "ssh", Host: host, CWD: cwd}
+}
+
+// Dial connects to a tool server at the given address.
+// Options (WithStrict, WithBuiltins, etc.) are applied to local servers.
+//
+// TODO: Unify local and remote behind a single protocol. Local execution
+// should eventually go through a Unix socket (same JSON-RPC as remote),
+// making the agent always a client. The elevation broker already proves
+// this pattern. Benefits: process isolation, shared tool server across
+// sessions, independent lifecycle. Transport becomes the only difference:
+//   - Network "local" → unix socket to co-located toolsrv process
+//   - Network "ssh"   → SSH tunnel to remote toolsrv process
+//   - Network "unix"  → pre-existing unix socket
+func Dial(ctx context.Context, addr Addr, opts ...Option) (Runner, error) {
+	switch addr.Network {
+	case "", "local":
+		srv := New(addr.CWD)
+		for _, o := range opts {
+			o(srv)
+		}
+		return srv, nil
+	case "ssh":
+		return RemoteDial(ctx, RemoteConfig{
+			SSHTarget: addr.Host,
+			CWD:       addr.CWD,
+		})
+	default:
+		return nil, fmt.Errorf("toolsrv.Dial: unsupported network %q", addr.Network)
+	}
+}