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.go

@@ -0,0 +1,66 @@
+// Package smtp provides a config wrapper around net/smtp for sending email.
+package smtp
+
+import (
+	"fmt"
+	"net/mail"
+	"net/smtp"
+)
+
+// SMTP holds configuration for sending emails via an SMTP server.
+type SMTP struct {
+	From     string `yaml:"from"`
+	Host     string `yaml:"host"`
+	Password string `yaml:"password"`
+	Port     string `yaml:"port"`
+	User     string `yaml:"user"`
+}
+
+// Address returns the host:port string.
+func (s *SMTP) Address() string {
+	return fmt.Sprintf("%s:%s", s.Host, s.Port)
+}
+
+// Auth creates a PlainAuth using the configured credentials.
+func (s *SMTP) Auth() smtp.Auth {
+	return smtp.PlainAuth("", s.User, s.Password, s.Host)
+}
+
+// Send emails the given recipients using net/smtp.
+// Refer to smtp.SendMail for its parameters and limitations.
+func (s *SMTP) Send(to []string, msg []byte) error {
+	if err := smtp.SendMail(
+		s.Address(),
+		s.Auth(),
+		s.From,
+		to,
+		msg,
+	); err != nil {
+		return fmt.Errorf("smtp send: %w", err)
+	}
+
+	return nil
+}
+
+// Validate checks that all required configuration fields are set.
+func (s *SMTP) Validate() error {
+	if s.From == "" {
+		return fmt.Errorf("missing from")
+	}
+	if _, err := mail.ParseAddress(s.From); err != nil {
+		return fmt.Errorf("invalid from: %w", err)
+	}
+	if s.Host == "" {
+		return fmt.Errorf("missing host")
+	}
+	if s.Password == "" {
+		return fmt.Errorf("missing password")
+	}
+	if s.Port == "" {
+		return fmt.Errorf("missing port")
+	}
+	if s.User == "" {
+		return fmt.Errorf("missing user")
+	}
+	return nil
+}