feat(infer): messages starting with "other:" are not monthly fees

When the bank message starts with "other:" (case-insensitive),
inference does not look for a month, neither in the text nor from the
payment date. The trimmed message is written to Purpose, so reconcile
books the payment as an "other" transaction for the person instead of
a monthly fee. Messages containing "other:" elsewhere are unchanged.

- matching.IsOtherPurpose / is_other_purpose shared by inference and
  reconcile
- reconcile applies the rule to rows not yet processed by inference
- no person found: Purpose is still written, row shows as unmatched
- implemented in Go and legacy Python; four new parity fixtures

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go/internal/domain/reconcile/reconcile.go

@@ -203,7 +203,7 @@ func Reconcile(
 		personStr := strings.TrimSpace(tx.Person)
 		purposeStr := strings.TrimSpace(tx.Purpose)
 		personStr = questionMarkRe.ReplaceAllString(personStr, "")
-		isOther := strings.HasPrefix(strings.ToLower(purposeStr), "other:")
+		isOther := matching.IsOtherPurpose(purposeStr)
 
 		var matchedMembers []matching.Match
 		var matchedMonths []string
@@ -249,6 +249,11 @@ func Reconcile(
 			matchedMembers = inferred.Members
 			matchedMonths = inferred.Months
 			amount = tx.Amount
+			// A message starting with "other:" is inferred as a non-fee payment.
+			if len(matchedMonths) == 1 && matching.IsOtherPurpose(matchedMonths[0]) {
+				isOther = true
+				purposeStr = matchedMonths[0]
+			}
 		}
 
 		if len(matchedMembers) == 0 || len(matchedMonths) == 0 {