feat: add --parent flag to bd list command (bd-yqhh)

Filters issues by parent issue ID using parent-child dependencies.

Example: bd list --parent=bd-xyz --status=open

Changes:
- Add ParentID field to IssueFilter type
- Add --parent flag to list command
- Forward parent filter through RPC
- Implement filtering in SQLite and memory storage

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

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

internal/rpc/protocol.go

@@ -162,6 +162,9 @@ type ListArgs struct {
 
 	// Template filtering (beads-1ra)
 	IncludeTemplates bool `json:"include_templates,omitempty"`
+
+	// Parent filtering (bd-yqhh)
+	ParentID string `json:"parent_id,omitempty"`
 }
 
 // CountArgs represents arguments for the count operation

internal/rpc/server_issues_epics.go

@@ -751,6 +751,11 @@ func (s *Server) handleList(req *Request) Response {
 		filter.IsTemplate = &isTemplate
 	}
 
+	// Parent filtering (bd-yqhh)
+	if listArgs.ParentID != "" {
+		filter.ParentID = &listArgs.ParentID
+	}
+
 	// Guard against excessive ID lists to avoid SQLite parameter limits
 	const maxIDs = 1000
 	if len(filter.IDs) > maxIDs {