Tasks

• io-imap: add the connect options struct carrying starttls, auto-ID and the SASL-IR override; honour the override in connect instead of reading the capability unconditionally.
• src/config.rs: add the imap.sasl-ir field.
• src/imap/client.rs, src/account/check.rs and src/wizard/imap_smtp.rs: pass the override through the new connect options.
• Document the option in config.sample.toml and both changelogs.
• Build/test/fmt.
• Fold into cairn/spec/provider-quirks.md (cairn/spec/config.md holds no per-backend keys, so nothing to fold there); log; land.