feat: add --parent flag to bd update for reparenting issues (bd-cj2e)

Allows reparenting issues to a different epic/parent:
  bd update bd-xyz --parent=bd-epic

Implementation:
- Add Parent field to UpdateArgs in protocol.go
- Handle reparenting in RPC handler (server_issues_epics.go)
- Add --parent flag to updateCmd with both daemon and direct mode support
- Remove old parent-child dependency before adding new one
- Pass empty string to remove parent entirely

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

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

internal/rpc/protocol.go

@@ -124,6 +124,8 @@ type UpdateArgs struct {
 	SupersededBy *string `json:"superseded_by,omitempty"` // Replacement issue ID if obsolete
 	// Pinned field (bd-iea)
 	Pinned *bool `json:"pinned,omitempty"` // If true, issue is a persistent context marker
+	// Reparenting field (bd-cj2e)
+	Parent *string `json:"parent,omitempty"` // New parent issue ID (reparents the issue)
 }
 
 // CloseArgs represents arguments for the close operation