feat(04-03): select memory, log, and SMTP mail drivers from config

- Load mail.* and SUMMER_MAIL__ overrides to pick memory, log, or smtp
- Log headers and text without credentials; SMTP uses explicit TLS and go-mail
- Reject CR/LF headers and invalid addresses and surface driver errors once

Co-authored-by: Cursor <cursoragent@cursor.com>

examples/hello/config/mail.yaml

@@ -0,0 +1,9 @@
+driver: memory
+from: Hello <hello@example.test>
+smtp:
+  host: 127.0.0.1
+  port: 1025
+  username: ""
+  password: ""
+  tls: none
+  timeout: 10s