Tasks

• Split messages, drafts, threads, labels, attachments, history, profile and settings into a sibling module plus one file per subcommand.
• Split the nine settings resources the same way.
• Gather the three copies of FormatArg into src/gmail/format.rs.
• Rename the twelve bare-verb command types to GmailSettings<Target><Verb>Command; give the send-as commands their Settings target.
• Replace the local read_message with the shared MessageArg in messages send/import/insert and drafts create/update; delete src/gmail/input.rs.
• Lift LabelsTable::new and the filter summaries out of their duplicated call sites.
• Remove blank lines between struct fields and enum variants across the subtree (inline-003).
• Update the schema registry paths; verify the CLI surface and all schemas are unchanged.
• Build/test/fmt/clippy.
• Fold into cairn/spec/commands.md; log; land.