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()`.

docs/plans/2026-05-07-1033-fuj-sync-dry-run.md

@@ -0,0 +1,36 @@
+# Plan: add `--dry-run` to `fuj sync`
+
+## Context
+
+`fuj infer` already supports `--dry-run` (it builds the planned `BatchUpdateValues`
+operations, prints them, and skips the actual write — see
+`go/internal/services/banksync/infer.go:136-156` and the
+`Dry run: would update N row(s).` line in `go/cmd/fuj/main.go:209-213`).
+
+`fuj sync` had no equivalent. It always committed three potential writes to the
+payments sheet: `WriteHeader` (if the header row is missing/wrong), `AppendValues`
+(for each new Fio transaction), and `SortByDateColumn` (if `--sort`, default true).
+For inspecting what a sync *would* do — useful when debugging dedupe, sanity-checking
+a date window, or wiring up the command for the first time on a new account — the
+only options were pointing at a throwaway spreadsheet or reading the diff after the fact.
+
+This change mirrors `infer`'s read-only mode for `sync`: same flag name, same output
+style, same "build the data structures, print instead of writing" shape.
+
+## Files modified
+
+1. `go/internal/services/banksync/sync.go` — `DryRun bool` field added to `SyncOpts`; three write points gated on `opts.DryRun`
+2. `go/cmd/fuj/main.go` — `--dry-run` flag added to `syncCmd`; final println split on `*dryRun`
+3. `go/internal/services/banksync/sync_test.go` — `TestSyncToSheets_DryRun` added
+4. `CHANGELOG.md` — entry added
+
+## Behaviour
+
+When `--dry-run` is set:
+
+- If the sheet header is missing/wrong → prints `Dry run: would write header row`; skips `WriteHeader`
+- For each non-deduped Fio transaction → prints `Dry run: would append date=… amount=… sender=… vs=… message=…`; skips `AppendValues`
+- If `--sort` is true → prints `Dry run: would sort by date`; skips `SortByDateColumn`
+- Returns `len(newRows)` so the caller can print `Dry run: would sync N new transaction(s).`
+
+The existing ID-dedup logic runs in full even during dry-run (reads the sheet, builds `existingIDs`), so the output reflects exactly what the next real sync would do.

go/cmd/fuj/main.go

@@ -134,8 +134,9 @@ func syncCmd(args []string) {
 	fromStr := fs.String("from", "", "start date YYYY-MM-DD")
 	toStr := fs.String("to", "", "end date YYYY-MM-DD")
 	sort := fs.Bool("sort", true, "sort sheet by date after appending")
+	dryRun := fs.Bool("dry-run", false, "print planned writes without modifying the sheet")
 	fs.Usage = func() {
-		fmt.Fprintln(os.Stderr, "usage: fuj sync [--days N] [--from YYYY-MM-DD --to YYYY-MM-DD] [--sort]")
+		fmt.Fprintln(os.Stderr, "usage: fuj sync [--days N] [--from YYYY-MM-DD --to YYYY-MM-DD] [--sort] [--dry-run]")
 		fs.PrintDefaults()
 	}
 	if err := fs.Parse(args); err != nil {
@@ -153,7 +154,7 @@ func syncCmd(args []string) {
 	}
 	fioCli := fio.New(cfg.FioAPIToken, config.IBANAccountNum(cfg.BankAccount), nil)
 
-	opts := banksync.SyncOpts{Days: *days, Sort: *sort}
+	opts := banksync.SyncOpts{Days: *days, Sort: *sort, DryRun: *dryRun}
 	if *fromStr != "" && *toStr != "" {
 		opts.From, err = time.Parse("2006-01-02", *fromStr)
 		if err != nil {
@@ -172,7 +173,11 @@ func syncCmd(args []string) {
 		fmt.Fprintf(os.Stderr, "fuj sync: %v\n", err)
 		os.Exit(1)
 	}
+	if *dryRun {
+		fmt.Printf("Dry run: would sync %d new transaction(s).\n", n)
+	} else {
 		fmt.Printf("Synced %d new transaction(s).\n", n)
+	}
 }
 
 func inferCmd(args []string) {

go/internal/services/banksync/sync.go

@@ -30,6 +30,7 @@ type SyncOpts struct {
 	Days     int       // look-back window when From/To are zero
 	From, To time.Time // explicit window (overrides Days)
 	Sort     bool      // sort the sheet by Date after appending
+	DryRun   bool      // print planned writes without modifying the sheet
 }
 
 // SyncToSheets fetches Fio transactions and appends new ones to the payments sheet.
@@ -52,9 +53,13 @@ func SyncToSheets(
 	if len(rows) > 0 {
 		header := rows[0]
 		if !headerMatches(header) {
+			if opts.DryRun {
+				fmt.Println("Dry run: would write header row")
+			} else {
 				if err := sh.WriteHeader(ctx, spreadsheetID, columnLabels); err != nil {
 					return 0, fmt.Errorf("sync: write header: %w", err)
 				}
+			}
 		} else {
 			for _, row := range rows[1:] {
 				if len(row) > 10 {
@@ -113,6 +118,17 @@ func SyncToSheets(
 		return 0, nil
 	}
 
+	if opts.DryRun {
+		for _, row := range newRows {
+			fmt.Printf("Dry run: would append date=%v amount=%v sender=%v vs=%v message=%v\n",
+				row[0], row[1], row[6], row[7], row[8])
+		}
+		if opts.Sort {
+			fmt.Println("Dry run: would sort by date")
+		}
+		return len(newRows), nil
+	}
+
 	if err := sh.AppendValues(ctx, spreadsheetID, "A2", newRows); err != nil {
 		return 0, fmt.Errorf("sync: append: %w", err)
 	}

go/internal/services/banksync/sync_test.go

@@ -127,6 +127,23 @@ func TestSyncToSheets_ExplicitDateWindow(t *testing.T) {
 	}
 }
 
+func TestSyncToSheets_DryRun(t *testing.T) {
+	sh := &sheets.Fake{Values: map[string][][]any{"SHEETID/A1:K": {}}}
+	fioFake := &fio.Fake{Transactions: testFioTxns}
+
+	n, err := SyncToSheets(context.Background(), "SHEETID", fioFake, sh,
+		SyncOpts{Days: 30, Sort: true, DryRun: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 2 {
+		t.Errorf("want 2 planned, got %d", n)
+	}
+	if len(sh.Appended) != 0 {
+		t.Error("dry-run must not call AppendValues")
+	}
+}
+
 // syncIDFor mirrors what SyncToSheets computes for a given fio.Transaction.
 func syncIDFor(tx fio.Transaction) string {
 	currency := tx.Currency