feat(cmail): add --cc/--bcc and threading headers to cmail-action send

cmail-action send couldn't do a proper reply (no Cc/Bcc, no In-Reply-To/References), so an org-drill session that needed to reply to an upstream maintainer hand-rolled a raw MIME message through msmtp instead. I extended build_message (the pure function) with cc, bcc, in_reply_to, and references, wired the matching --cc/--bcc (repeatable), --in-reply-to, and --references flags through cmd_send, and wrote the tests first. send_message derives recipients from the To/Cc/Bcc headers and strips Bcc, so no manual recipient list is needed.

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