feat(go): add --dry-run to fuj sync

Mirror fuj infer's read-only mode: SyncOpts.DryRun skips WriteHeader,
AppendValues, and SortByDateColumn, printing one "Dry run: would …"
line per planned operation instead. ID-dedup still runs so the output
reflects exactly what the next real sync would write.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## 2026-05-07 10:32 CEST — feat(go): --dry-run for fuj sync
+
+- `SyncOpts.DryRun bool` added; when true, `SyncToSheets` prints planned writes (`would write header row`, `would append date=… amount=… sender=…`, `would sort by date`) and returns without calling `WriteHeader`, `AppendValues`, or `SortByDateColumn`.
+- `fuj sync --dry-run` flag wired in `cmd/fuj/main.go`; mirrors existing `fuj infer --dry-run` behaviour.
+- `TestSyncToSheets_DryRun` added to banksync test suite.
+
 ## 2026-05-07 01:03 CEST — feat(go/M4): IO layer behind interfaces
 
 - `go/internal/io/attendance`: CSV-over-public-URL client + `Fake` for both adult and junior tabs.
@@ -36,6 +42,7 @@
 - `scripts/infer_payments.py`: union adults + junior rosters so junior-only members are visible to the matcher.
 - Root cause: `get_members_with_fees()` reads only the adults sheet; junior-only kids like Jáchym Kubík were absent from `member_names`, causing the exact-match short-circuit to never fire and a different adult sharing the first name to win via fuzzy review.
 - Two regression tests added to `tests/test_match_members.py`.
+
 ## 2026-05-06 16:05 CEST — feat(go/M2.10): port domain/reconcile.Reconcile
 
 - New `go/internal/domain/reconcile` package porting the three-phase payment allocation from `scripts/match_payments.py reconcile()`.