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/compact.go

@@ -953,6 +953,7 @@ func pruneExpiredTombstones(customTTL time.Duration) (*TombstonePruneResult, err
 			// Skip corrupt lines
 			continue
 		}
+		issue.SetDefaults() // Apply defaults for omitted fields (beads-399)
 		allIssues = append(allIssues, &issue)
 	}
 	if err := file.Close(); err != nil {
@@ -1049,6 +1050,7 @@ func previewPruneTombstones(customTTL time.Duration) (*TombstonePruneResult, err
 			// Skip corrupt lines
 			continue
 		}
+		issue.SetDefaults() // Apply defaults for omitted fields (beads-399)
 		allIssues = append(allIssues, &issue)
 	}