feat(types): add omitempty to JSONL fields for smaller output

Add omitempty json tag to:
- Issue.Description: often empty for quick tasks
- Dependency.CreatedBy: often empty

This reduces JSONL file size when these fields are empty, leading to
faster syncs and smaller repository size.

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

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

internal/types/types.go

@@ -348,7 +348,7 @@ type Dependency struct {
 	DependsOnID string         `json:"depends_on_id"`
 	Type        DependencyType `json:"type"`
 	CreatedAt   time.Time      `json:"created_at"`
-	CreatedBy   string         `json:"created_by"`
+	CreatedBy   string         `json:"created_by,omitempty"`
 	// Metadata contains type-specific edge data (JSON blob)
 	// Examples: similarity scores, approval details, skill proficiency
 	Metadata string `json:"metadata,omitempty"`