Fix validatePostImport false positive for legitimate deletions (bd-pg1)

The sync validation was incorrectly triggering 'data loss detected' when
issue count decreased after import, even for legitimate deletions recorded
in deletions.jsonl.

Changes:
- Modified validatePostImport to accept jsonlPath and check deletions manifest
- When issue count decreases, check if decrease is within recorded deletions
- Updated all call sites in sync.go and daemon_sync.go
- Added comprehensive tests for deletion-aware validation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/sync.go

@@ -332,7 +332,7 @@ Use --merge to merge the sync branch back to main branch.`,
 						if err != nil {
 							fmt.Fprintf(os.Stderr, "Warning: failed to count issues after import: %v\n", err)
 						} else {
-							if err := validatePostImport(beforeCount, afterCount); err != nil {
+							if err := validatePostImport(beforeCount, afterCount, jsonlPath); err != nil {
 								fmt.Fprintf(os.Stderr, "Post-import validation failed: %v\n", err)
 								os.Exit(1)
 							}