feat: Add --assignee flag to template instantiate, fix mail inbox --identity

- Add --assignee flag to `bd template instantiate` for work delegation
  - Assigns the root epic to specified agent/user
  - Child issues retain their template-defined assignees
  - Enables Gas Town orchestration: instantiate + assign in one step

- Add missing --identity flag to `bd mail inbox` for consistency
  - All mail subcommands now support --identity override

- Delete obsolete HANDOFF-template-redesign.md (work complete)

- Add test for assignee override behavior

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

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

cmd/bd/mail.go

@@ -130,6 +130,7 @@ func init() {
 	// Inbox command flags
 	mailInboxCmd.Flags().StringVar(&mailFrom, "from", "", "Filter by sender")
 	mailInboxCmd.Flags().IntVar(&mailPriorityFlag, "priority", -1, "Filter by priority (0-4)")
+	mailInboxCmd.Flags().StringVar(&mailIdentity, "identity", "", "Override recipient identity")
 
 	// Read command flags
 	mailReadCmd.Flags().StringVar(&mailIdentity, "identity", "", "Override identity for access check")