Add comments feature (bd-162)

- Add comments table to SQLite schema
- Add Comment type to internal/types
- Implement AddIssueComment and GetIssueComments in storage layer
- Update JSONL export/import to include comments
- Add comments to 'bd show' output
- Create 'bd comments' CLI command structure
- Fix UpdateIssueID to update comments table and defer FK checks
- Add GetIssueComments/AddIssueComment to Storage interface

Note: CLI command needs daemon RPC support (tracked in bd-163)
Amp-Thread-ID: https://ampcode.com/threads/T-ece10dd1-cf64-48ff-9adb-dd304d0bcb25
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/sync.go

@@ -270,6 +270,15 @@ func exportToJSONL(ctx context.Context, jsonlPath string) error {
 		issue.Labels = labels
 	}
 
+	// Populate comments for all issues
+	for _, issue := range issues {
+		comments, err := store.GetIssueComments(ctx, issue.ID)
+		if err != nil {
+			return fmt.Errorf("failed to get comments for %s: %w", issue.ID, err)
+		}
+		issue.Comments = comments
+	}
+
 	// Create temp file for atomic write
 	dir := filepath.Dir(jsonlPath)
 	base := filepath.Base(jsonlPath)