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/types/types.go

@@ -562,6 +562,9 @@ type IssueFilter struct {
 
 	// Template filtering (beads-1ra)
 	IsTemplate *bool // Filter by template flag (nil = any, true = only templates, false = exclude templates)
+
+	// Parent filtering (bd-yqhh): filter children by parent issue ID
+	ParentID *string // Filter by parent issue (via parent-child dependency)
 }
 
 // SortPolicy determines how ready work is ordered