docs: Add XML documentation to IEnvironmentVariables interface

Fixes #1522

Add comprehensive XML documentation to the IEnvironmentVariables interface:
- Interface summary describing its purpose for reading/modifying environment variables
- GetEnvironmentVariable: document name, target params and nullable return
- GetEnvironmentVariables: document target param and dictionary return
- SetEnvironmentVariable: document variableName, value, and target params
- GetPath: document target param and list return with platform delimiter info
- AddToPath: document pathToAdd and target params with platform delimiter info

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: thomhurst

src/ModularPipelines/Context/IEnvironmentVariables.cs

@@ -1,14 +1,74 @@
 namespace ModularPipelines.Context;
 
+/// <summary>
+/// Provides functionality for reading and modifying environment variables.
+/// This interface abstracts access to system environment variables, allowing retrieval
+/// and modification of variables at different scopes (process, user, or machine level).
+/// </summary>
 public interface IEnvironmentVariables
 {
+    /// <summary>
+    /// Gets the value of the specified environment variable.
+    /// </summary>
+    /// <param name="name">The name of the environment variable to retrieve.</param>
+    /// <param name="target">
+    /// The target scope from which to retrieve the environment variable.
+    /// Defaults to <see cref="EnvironmentVariableTarget.Process"/>.
+    /// </param>
+    /// <returns>
+    /// The value of the environment variable specified by <paramref name="name"/>,
+    /// or <c>null</c> if the environment variable is not found.
+    /// </returns>
     string? GetEnvironmentVariable(string name, EnvironmentVariableTarget target = EnvironmentVariableTarget.Process);
 
+    /// <summary>
+    /// Gets all environment variables as a dictionary.
+    /// </summary>
+    /// <param name="target">
+    /// The target scope from which to retrieve environment variables.
+    /// Defaults to <see cref="EnvironmentVariableTarget.Process"/>.
+    /// </param>
+    /// <returns>
+    /// A dictionary containing all environment variable names and their values
+    /// for the specified target scope.
+    /// </returns>
     IDictionary<string, string> GetEnvironmentVariables(EnvironmentVariableTarget target = EnvironmentVariableTarget.Process);
 
+    /// <summary>
+    /// Sets the value of an environment variable.
+    /// </summary>
+    /// <param name="variableName">The name of the environment variable to set.</param>
+    /// <param name="value">The value to assign to the environment variable.</param>
+    /// <param name="target">
+    /// The target scope in which to set the environment variable.
+    /// Defaults to <see cref="EnvironmentVariableTarget.Process"/>.
+    /// </param>
     void SetEnvironmentVariable(string variableName, string value, EnvironmentVariableTarget target = EnvironmentVariableTarget.Process);
 
+    /// <summary>
+    /// Gets the PATH environment variable as a list of individual paths.
+    /// The PATH variable is split using the platform-appropriate delimiter
+    /// (semicolon on Windows, colon on Unix-like systems).
+    /// </summary>
+    /// <param name="target">
+    /// The target scope from which to retrieve the PATH variable.
+    /// Defaults to <see cref="EnvironmentVariableTarget.Process"/>.
+    /// </param>
+    /// <returns>
+    /// A read-only list of paths from the PATH environment variable.
+    /// Returns an empty list if the PATH variable is not set.
+    /// </returns>
     IReadOnlyList<string> GetPath(EnvironmentVariableTarget target = EnvironmentVariableTarget.Process);
 
+    /// <summary>
+    /// Adds a path to the PATH environment variable.
+    /// The path is appended using the platform-appropriate delimiter
+    /// (semicolon on Windows, colon on Unix-like systems).
+    /// </summary>
+    /// <param name="pathToAdd">The path to add to the PATH environment variable.</param>
+    /// <param name="target">
+    /// The target scope in which to modify the PATH variable.
+    /// Defaults to <see cref="EnvironmentVariableTarget.Process"/>.
+    /// </param>
     void AddToPath(string pathToAdd, EnvironmentVariableTarget target = EnvironmentVariableTarget.Process);
 }