fix: recover orphaned mail migration to bd v0.32.0

The merge at 96c773f lost changes from 5791752 (beads-sync branch).
Re-implementing:
- router.go: Use bd create --type=message with labels
- mailbox.go: Use bd list/show/close instead of bd mail commands
- types.go: Add Pinned field to Message struct

Original work was in commits 5791752 and 4c060f4 on beads-sync.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mail/types.go

@@ -94,6 +94,9 @@ type Message struct {
 
 	// ReplyTo is the ID of the message this is replying to.
 	ReplyTo string `json:"reply_to,omitempty"`
+
+	// Pinned marks the message as pinned (won't be auto-archived).
+	Pinned bool `json:"pinned,omitempty"`
 }
 
 // NewMessage creates a new message with a generated ID and thread ID.