fix(autoflush): preserve comments during full re-export triggered by hash mismatch (#1039)

Problem:
When a JSONL file hash mismatch was detected (e.g., after git operations
that modify the JSONL without updating export_hashes), the autoflush
system would trigger a full re-export. During this re-export, all issue
comments were silently dropped from the exported JSONL file.

Steps to reproduce:
1. Have a beads database with issues containing comments
2. Create a situation where JSONL hash doesn't match stored hash
   (e.g., clone a repo, or manual JSONL edits)
3. Run any bd command that triggers autoflush (e.g., `bd create foo`)
4. Observe warning: "JSONL file hash mismatch detected"
5. Check .beads/issues.jsonl - all comments are now missing

Root cause:
Two different export code paths existed:
- exportToJSONLWithStore (daemon_sync.go) - correctly populated comments
- fetchAndMergeIssues (autoflush.go) - only fetched dependencies, NOT comments

When hash mismatch triggered a full re-export via the autoflush path,
fetchAndMergeIssues was called but it never populated issue.Comments,
resulting in all comments being lost.

Fix:
Add GetIssueComments call in fetchAndMergeIssues to populate comments
for each issue before export, matching the behavior of exportToJSONLWithStore.

Note: Labels were not affected because GetIssue() already populates them
internally. Comments are stored in a separate table and require explicit
fetching via GetIssueComments().

cmd/bd/autoflush.go

@@ -626,6 +626,13 @@ func fetchAndMergeIssues(ctx context.Context, s storage.Storage, dirtyIDs []stri
 		}
 		issue.Dependencies = deps
 
+		// Get comments for this issue
+		comments, err := s.GetIssueComments(ctx, issueID)
+		if err != nil {
+			return fmt.Errorf("failed to get comments for %s: %w", issueID, err)
+		}
+		issue.Comments = comments
+
 		// Update map
 		issueMap[issueID] = issue
 	}