feat(jsonl): add omitempty to reduce JSONL bloat (beads-399)

Add omitempty JSON tags to Issue struct fields (Description, Status,
Priority, IssueType) and SetDefaults method to apply proper defaults
when importing JSONL with omitted fields.

This reduces JSONL file size for minimal issues like notifications
by not exporting empty/default values.

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

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

cmd/bd/import.go

@@ -205,6 +205,7 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 				fmt.Fprintf(os.Stderr, "Error parsing line %d: %v\n", lineNum, err)
 				os.Exit(1)
 			}
+			issue.SetDefaults() // Apply defaults for omitted fields (beads-399)
 
 			allIssues = append(allIssues, &issue)
 		}