Add --parent flag support in daemon mode (bd-2e94)

- Added Parent field to CreateArgs RPC protocol
- Updated CLI to pass parent ID to daemon instead of erroring
- Added parent ID handling in RPC server to call GetNextChildID
- Added validation to prevent both --id and --parent flags
- Added comprehensive tests for hierarchical child creation
- Resolves error: '--parent flag not yet supported in daemon mode'

Amp-Thread-ID: https://ampcode.com/threads/T-3e0f76df-4ba6-4b16-bf75-bb7ea6b19541
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/protocol.go

@@ -57,6 +57,7 @@ type Response struct {
 // CreateArgs represents arguments for the create operation
 type CreateArgs struct {
 	ID                 string   `json:"id,omitempty"`
+	Parent             string   `json:"parent,omitempty"` // Parent ID for hierarchical issues
 	Title              string   `json:"title"`
 	Description        string   `json:"description,omitempty"`
 	IssueType          string   `json:"issue_type"`