Store outbound RFC Message-ID + reply-all + thread view

[longevityboris]

Problem

• Sent messages don't store their outbound RFC Message-ID (Resend returns it but we don't capture it)
• Reply-to-sent is intentionally blocked because of this
• No reply-all (only reply-to-sender)
• No conversation/thread grouping view

Proposal

1. Capture Message-ID from Resend response on send
2. Enable reply-to-sent messages
3. Add reply --all flag that includes all original recipients
4. Add inbox thread <id> command that groups messages by References chain

Priority: MEDIUM