Fix: Preserve timestamps during import (GH-121)

- Change validateBatchIssues() to only set timestamps if IsZero()
- Preserves historical timestamps from external systems (Jira, GitHub)
- Fixes dirty git repo after importing unchanged JSONL
- New issues still get current timestamps as before
- Add daemon.lock to .gitignore

Closes bd-55
Fixes #121

Amp-Thread-ID: https://ampcode.com/threads/T-e53c4a96-38dd-440a-9b8d-824992d33a40
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/sqlite.go

@@ -665,8 +665,13 @@ func validateBatchIssues(issues []*types.Issue) error {
 			return fmt.Errorf("issue %d is nil", i)
 		}
 
-		issue.CreatedAt = now
-		issue.UpdatedAt = now
+		// Only set timestamps if not already provided
+		if issue.CreatedAt.IsZero() {
+			issue.CreatedAt = now
+		}
+		if issue.UpdatedAt.IsZero() {
+			issue.UpdatedAt = now
+		}
 
 		if err := issue.Validate(); err != nil {
 			return fmt.Errorf("validation failed for issue %d: %w", i, err)