Add answer worker

Subsystem for message processing: parses messages, generates LLM
responses, and replies with SMTP.

Introduces:
- answer: message processing worker.
- llm: OpenAI API compatible client with support for tool execution.
- message: message parsing and response logic.
- tool: converts YAML configuration into executable subprocesses.
- smtp: simple config and client wrapper for sending email.

internal/smtp/smtp_test.go

@@ -0,0 +1,111 @@
+package smtp
+
+import (
+	"testing"
+)
+
+func TestAddress(t *testing.T) {
+	s := &SMTP{
+		Host: "smtp.example.com",
+		Port: "587",
+	}
+	expected := "smtp.example.com:587"
+	if got := s.Address(); got != expected {
+		t.Errorf("Address() = %q, want %q", got, expected)
+	}
+}
+
+func TestAuth(t *testing.T) {
+	s := &SMTP{
+		User:     "user",
+		Password: "password",
+		Host:     "smtp.example.com",
+	}
+
+	// Verify return of non-nil Auth mechanism.
+	if got := s.Auth(); got == nil {
+		t.Error("Auth() returned nil")
+	}
+}
+
+func TestValidate(t *testing.T) {
+	tests := []struct {
+		name        string
+		s           *SMTP
+		shouldError bool
+	}{
+		{
+			name: "valid configuration",
+			s: &SMTP{
+				From:     "sender@example.com",
+				Host:     "smtp.example.com",
+				Password: "secretpassword",
+				Port:     "587",
+				User:     "user",
+			},
+			shouldError: false,
+		},
+		{
+			name: "missing from",
+			s: &SMTP{
+				Host:     "smtp.example.com",
+				Password: "p",
+				Port:     "587",
+				User:     "u",
+			},
+			shouldError: true,
+		},
+		{
+			name: "missing host",
+			s: &SMTP{
+				From:     "f",
+				Password: "p",
+				Port:     "587",
+				User:     "u",
+			},
+			shouldError: true,
+		},
+		{
+			name: "missing password",
+			s: &SMTP{
+				From: "f",
+				Host: "h",
+				Port: "587",
+				User: "u",
+			},
+			shouldError: true,
+		},
+		{
+			name: "missing port",
+			s: &SMTP{
+				From:     "f",
+				Host:     "h",
+				Password: "p",
+				User:     "u",
+			},
+			shouldError: true,
+		},
+		{
+			name: "missing user",
+			s: &SMTP{
+				From:     "f",
+				Host:     "h",
+				Password: "p",
+				Port:     "587",
+			},
+			shouldError: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.s.Validate()
+			if tt.shouldError && err == nil {
+				t.Fatalf("expected error, got nil")
+			}
+			if !tt.shouldError && err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+		})
+	}
+}