Adopt the shared .golangci.yml and fix the code to it (closes #6)

Vendor .golangci.yml byte-identical from sneak/prompts at cc440118 and
move the Dockerfile lint phase to golangci-lint v2.14.0 by the digest
REPO_POLICIES.md names. Fix the code to that config with flags, help
text, output files, SQL and the order of steps unchanged; long
functions are split into named steps.

Judgement call: the extraction transaction is now rolled back on every
early return; the old deferred rollback missed most failures and could
dereference a nil transaction.
Wording change: the reversed-range error reads "--from is after --to
(--from X, --to Y)", since err113 requires a wrapped sentinel.
Ten //nolint directives (gosec, mnd, unqueryvet), each with its reason.

Model: opus-5-5

cmd/bsdaily/main.go

@@ -1,6 +1,9 @@
+// Package main is the bsdaily command. It extracts one day, or a range
+// of days, from the latest daily snapshot.
 package main
 
 import (
+	"errors"
 	"fmt"
 	"log/slog"
 	"os"
@@ -10,75 +13,113 @@ import (
 	"github.com/spf13/cobra"
 )
 
+var (
+	errDateExclusive  = errors.New("--date and --from/--to are mutually exclusive")
+	errFromRequiresTo = errors.New("--from requires --to")
+	errToRequiresFrom = errors.New("--to requires --from")
+	errFromAfterTo    = errors.New("--from is after --to")
+)
+
 func main() {
 	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
 		Level: slog.LevelInfo,
 	}))
 	slog.SetDefault(logger)
 
-	var dateFlag string
-	var fromFlag string
-	var toFlag string
+	var dateFlag, fromFlag, toFlag string
 
 	rootCmd := &cobra.Command{
 		Use:          "bsdaily",
 		Short:        "Extract a single day's data from the latest daily snapshot",
 		SilenceUsage: true,
-		RunE: func(cmd *cobra.Command, args []string) error {
-			hasDate := dateFlag != ""
-			hasFrom := fromFlag != ""
-			hasTo := toFlag != ""
-
-			// Validate mutual exclusivity
-			if hasDate && (hasFrom || hasTo) {
-				return fmt.Errorf("--date and --from/--to are mutually exclusive")
-			}
-			if hasFrom != hasTo {
-				if hasFrom {
-					return fmt.Errorf("--from requires --to")
-				}
-				return fmt.Errorf("--to requires --from")
-			}
-
-			var targetDates []time.Time
-
-			if hasDate {
-				t, err := time.Parse("2006-01-02", dateFlag)
-				if err != nil {
-					return fmt.Errorf("invalid --date %q (expected YYYY-MM-DD): %w", dateFlag, err)
-				}
-				targetDates = []time.Time{t}
-			} else if hasFrom {
-				from, err := time.Parse("2006-01-02", fromFlag)
-				if err != nil {
-					return fmt.Errorf("invalid --from %q (expected YYYY-MM-DD): %w", fromFlag, err)
-				}
-				to, err := time.Parse("2006-01-02", toFlag)
-				if err != nil {
-					return fmt.Errorf("invalid --to %q (expected YYYY-MM-DD): %w", toFlag, err)
-				}
-				if from.After(to) {
-					return fmt.Errorf("--from %s is after --to %s", fromFlag, toFlag)
-				}
-				for d := from; !d.After(to); d = d.AddDate(0, 0, 1) {
-					targetDates = append(targetDates, d)
-				}
-			}
-			// else: targetDates remains nil → Run() defaults to snapshot date minus one
-
-			if err := bsdaily.Run(targetDates); err != nil {
+		RunE: func(_ *cobra.Command, _ []string) error {
+			targetDates, err := parseTargetDates(dateFlag, fromFlag, toFlag)
+			if err != nil {
 				return err
 			}
+
+			err = bsdaily.Run(targetDates)
+			if err != nil {
+				return err
+			}
+
 			slog.Info("completed successfully")
+
 			return nil
 		},
 	}
 
-	rootCmd.Flags().StringVarP(&dateFlag, "date", "d", "", "target date to extract (YYYY-MM-DD); defaults to snapshot date minus one day")
-	rootCmd.Flags().StringVar(&fromFlag, "from", "", "start of date range to extract (YYYY-MM-DD, inclusive); use with --to")
-	rootCmd.Flags().StringVar(&toFlag, "to", "", "end of date range to extract (YYYY-MM-DD, inclusive); use with --from")
+	rootCmd.Flags().StringVarP(&dateFlag, "date", "d", "",
+		"target date to extract (YYYY-MM-DD); "+
+			"defaults to snapshot date minus one day")
+	rootCmd.Flags().StringVar(&fromFlag, "from", "",
+		"start of date range to extract (YYYY-MM-DD, inclusive); use with --to")
+	rootCmd.Flags().StringVar(&toFlag, "to", "",
+		"end of date range to extract (YYYY-MM-DD, inclusive); use with --from")
 
-	if err := rootCmd.Execute(); err != nil {
+	err := rootCmd.Execute()
+	if err != nil {
 		os.Exit(1)
 	}
 }
+
+// parseTargetDates turns the --date, --from and --to flags into the days
+// to extract. It returns nil when none of them is set, which Run takes
+// to mean the snapshot date minus one day.
+func parseTargetDates(dateFlag, fromFlag, toFlag string) ([]time.Time, error) {
+	hasDate := dateFlag != ""
+	hasFrom := fromFlag != ""
+	hasTo := toFlag != ""
+
+	// Validate mutual exclusivity
+	if hasDate && (hasFrom || hasTo) {
+		return nil, errDateExclusive
+	}
+
+	if hasFrom != hasTo {
+		if hasFrom {
+			return nil, errFromRequiresTo
+		}
+
+		return nil, errToRequiresFrom
+	}
+
+	if hasDate {
+		t, err := time.Parse("2006-01-02", dateFlag)
+		if err != nil {
+			return nil, fmt.Errorf(
+				"invalid --date %q (expected YYYY-MM-DD): %w", dateFlag, err)
+		}
+
+		return []time.Time{t}, nil
+	}
+
+	if !hasFrom {
+		return nil, nil
+	}
+
+	from, err := time.Parse("2006-01-02", fromFlag)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"invalid --from %q (expected YYYY-MM-DD): %w", fromFlag, err)
+	}
+
+	to, err := time.Parse("2006-01-02", toFlag)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"invalid --to %q (expected YYYY-MM-DD): %w", toFlag, err)
+	}
+
+	if from.After(to) {
+		return nil, fmt.Errorf("%w (--from %s, --to %s)",
+			errFromAfterTo, fromFlag, toFlag)
+	}
+
+	var targetDates []time.Time
+
+	for d := from; !d.After(to); d = d.AddDate(0, 0, 1) {
+		targetDates = append(targetDates, d)
+	}
+
+	return targetDates, nil
+}