Replace `bd mail send/inbox/read/ack` commands with `bd create/list/show/close`.
This separates the orchestration layer (gt) from the data plane (beads).
Changes:
- router.go: Use `bd create --type=message` instead of `bd mail send`
- mailbox.go: Use `bd list --type=message` and `bd show` for inbox/read
- types.go: Parse metadata from labels (from:, thread:, reply-to:)
- mail.go: Fix findBeadsWorkDir to prefer rig-level beads, fix crew address format
Messages are now stored as beads issues with type=message. Metadata (sender,
thread, reply-to) is stored in labels for retrieval.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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