Replace ad-hoc .env loading with standard configuration

[Copilot]

The CLI was manually parsing .env files to read OpenAI settings; align with standard .NET configuration/user secrets instead.

• Configuration source: use ConfigurationBuilder with user secrets and environment variables; remove custom .env parsing.
• CLI project: add UserSecretsId and configuration package references to support standard secret management.

Example usage:

var configuration = new ConfigurationBuilder()
    .AddUserSecrets<Program>(optional: true)
    .AddEnvironmentVariables()
    .Build();

var apiKey = configuration["OPENAI_API_KEY"];

Original prompt

I see that the AI tried to replicate Go .env files

var envPath = Path.Combine(Directory.GetCurrentDirectory(), ".env");
if (File.Exists(envPath))
{
try
{
foreach (var line in File.ReadAllLines(envPath))
{
var trimmed = line.Trim();
if (string.IsNullOrEmpty(trimmed) || trimmed.StartsWith('#'))
continue;

                var parts = trimmed.Split('=', 2);
                if (parts.Length == 2)
                {
                    Environment.SetEnvironmentVariable(parts[0].Trim(), parts[1].Trim());
                }
            }
        }
        catch (Exception ex)
        {
            await stderr.WriteLineAsync($"failed to load .env: {ex.Message}");
            return 1;
        }
    }

    var defaultModel = Environment.GetEnvironmentVariable("OPENAI_MODEL") ?? "gpt-4o";
    var defaultReasoning = Environment.GetEnvironmentVariable("OPENAI_REASONING_EFFORT") ?? "";
    var defaultBaseURL = Environment.GetEnvironmentVariable("OPENAI_BASE_URL") ?? "";

    string? model = null, reasoningEffort = null, promptAugmentation = null, baseURL = null, prompt = null, research = null;

just make it use dotnet secrets or something more standard

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[rogeralsing]

Nice cleanup: replacing the ad-hoc parser with + user-secrets/env vars is much more idiomatic.

A couple small follow-ups to consider:

• : the error message still says but it can now come from user-secrets too; suggest wording like .
• This change removes support entirely; if that’s intentional, a short README note (how to set via ) would prevent confusion.

FYI: I built the branch locally with MSBUILD : error MSB1003: Specify a project or solution file. The current working directory does not contain a project or solution file. successfully (existing warnings only).

[rogeralsing]

Nice cleanup: replacing the ad-hoc .env parser with ConfigurationBuilder + user-secrets/env vars is much more idiomatic.

A couple small follow-ups to consider:

• Program.cs: the error message still says OPENAI_API_KEY must be set in the environment. but it can now come from user-secrets too; suggest wording like must be configured (env var or user-secrets).
• This change removes .env support entirely; if that’s intentional, a short README note (how to set via dotnet user-secrets set OPENAI_API_KEY ... --project src/Asynkron.Agent.Cli) would prevent confusion.

FYI: I built the branch locally with dotnet build -c Release successfully (existing warnings only).

[rogeralsing]

Housekeeping: my first review ended up mangled due to shell quoting/backticks. Please ignore that one; the following review (with proper Markdown) has the actual feedback.