#968 Remove SMTP leftovers after #927: core config.ini [email], config.example.ini x2, preinstalled_apps.yaml default_secrets, aiosmtplib extra, CLAUDE.md SMTP sections

EARS SPEC: - The Highway repositories shall contain no SMTP host, SMTP username or Vault mail-secret reference in config, example config, app defaults or agent instructions. - The highway-engine package shall declare no SMTP client dependency. - If any process reads an [email] section after this change, then it shall be one that only reads from_address/from_name for mail-api. SYNTHESIS: localised chore; domain = config hygiene; concept = delete dead configuration (grep-verified consumers first). CONTEXT: mail-api key 95 revoked 2026-10-20T00:04:33Z and SMTP login highway-mail locked 2026-10-20T00:05:58Z (mail-api-f3dc60). Verified: SMTP AUTH with the real password returned 235 before the lock and 535 after.