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/llm/llm_test.go

@@ -0,0 +1,91 @@
+package llm
+
+import (
+	"testing"
+)
+
+func TestConfigValidate(t *testing.T) {
+	tests := []struct {
+		name    string
+		cfg     Config
+		wantErr bool
+	}{
+		{
+			name:    "empty config",
+			cfg:     Config{},
+			wantErr: true,
+		},
+		{
+			name: "missing model",
+			cfg: Config{
+				URL: "https://api.example.com",
+			},
+			wantErr: true,
+		},
+		{
+			name: "missing URL",
+			cfg: Config{
+				Model: "gpt-4o",
+			},
+			wantErr: true,
+		},
+		{
+			name: "invalid timeout",
+			cfg: Config{
+				Model:   "gpt-4o",
+				URL:     "https://api.example.com",
+				Timeout: "not-a-duration",
+			},
+			wantErr: true,
+		},
+		{
+			name: "valid minimal config",
+			cfg: Config{
+				Model: "gpt-4o",
+				URL:   "https://api.example.com",
+			},
+			wantErr: false,
+		},
+		{
+			name: "valid config with timeout",
+			cfg: Config{
+				Model:   "gpt-4o",
+				URL:     "https://api.example.com",
+				Timeout: "15m",
+			},
+			wantErr: false,
+		},
+		{
+			name: "valid config with complex timeout",
+			cfg: Config{
+				Model:   "gpt-4o",
+				URL:     "https://api.example.com",
+				Timeout: "1h30m45s",
+			},
+			wantErr: false,
+		},
+		{
+			name: "valid full config",
+			cfg: Config{
+				Key:          "sk-test-key",
+				Model:        "gpt-4o",
+				SystemPrompt: "You are a helpful assistant.",
+				Timeout:      "30m",
+				URL:          "https://api.example.com",
+			},
+			wantErr: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.cfg.Validate()
+			if (err != nil) != tt.wantErr {
+				t.Errorf(
+					"Validate() error = %v, wantErr %v",
+					err, tt.wantErr,
+				)
+			}
+		})
+	}
+}