Add dual-inbox architecture where ephemeral messages go to
.beads-wisp/.beads/ instead of .beads/. Lifecycle messages
(POLECAT_STARTED, NUDGE, etc.) auto-detect as ephemeral.
Changes:
- Add Ephemeral and Source fields to mail.Message
- Add --ephemeral flag to gt mail send
- Router auto-detects lifecycle message patterns
- Mailbox merges messages from both persistent and wisp storage
- Inbox displays (ephemeral) indicator for wisp messages
- Delete/archive works for both message types
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>