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/autoflush.go

@@ -165,6 +165,7 @@ func autoImportIfNewer() {
 			fmt.Fprintf(os.Stderr, "Auto-import skipped: parse error at line %d: %v\nSnippet: %s\n", lineNo, err, snippet)
 			return
 		}
+		issue.SetDefaults() // Apply defaults for omitted fields (beads-399)
 
 		// Fix closed_at invariant: closed issues must have closed_at timestamp
 		if issue.Status == types.StatusClosed && issue.ClosedAt == nil {
@@ -662,6 +663,7 @@ func flushToJSONLWithState(state flushState) {
 				}
 				var issue types.Issue
 				if err := json.Unmarshal([]byte(line), &issue); err == nil {
+					issue.SetDefaults() // Apply defaults for omitted fields (beads-399)
 					issueMap[issue.ID] = &issue
 				} else {
 					// Warn about malformed JSONL lines