Reject a duration with characters outside its parts (closes #215)

parseDuration fell back to an unanchored search for number-and-unit
pieces when time.ParseDuration failed, and skipped everything in
between. 1.0y became 0, so `snapshot create --prune --keep-newer-than
1.0y` deleted every snapshot of the backed-up names, the new one
included. 2.1w became one week and 1,5y five years. The fallback now
requires the whole input to be whole-number-and-unit parts with nothing
between them. A bare number is rejected before time.ParseDuration sees
it, since Go reads 0 and +0 as zero with no unit.

Judgement call: a space between number and unit (`30 days`) was
accepted and is now an error, matching Go's own units.

Model: opus-5-5

TODO.md

@@ -28,8 +28,8 @@ the tag exists and is exercised; what is left is merging `next` to
   parser picked out the parts it recognised and skipped the rest, so
   `1.0y` became zero and `--prune --keep-newer-than 1.0y` deleted every
   snapshot of the backed-up names, the new one included. `1.0y`,
-  `1,5y`, `30 days` and `x7d` are now errors; decimals still work in Go
-  units such as `1.5h`.
+  `1,5y`, `30 days`, `x7d` and a bare `0` are now errors; decimals still
+  work in Go units such as `1.5h`.
 
 - 2026-10-06: Made a backup re-chunk a known file that lists a chunk no
   uploaded blob holds

internal/vaultik/helpers.go

@@ -149,6 +149,13 @@ func parseDuration(s string) (time.Duration, error) {
 		return 0, errNegativeDuration
 	}
 
+	// A bare number has no unit, but time.ParseDuration accepts 0, which
+	// would put the cutoff at now and select every snapshot.
+	_, err := strconv.ParseFloat(s, 64)
+	if err == nil {
+		return 0, fmt.Errorf("%w: %q", errInvalidDuration, s)
+	}
+
 	d, err := time.ParseDuration(s)
 	if err == nil {
 		return d, nil

internal/vaultik/helpers_test.go

@@ -74,6 +74,8 @@ func TestParseDuration(t *testing.T) {
 		{"1y6mo", 365*24*time.Hour + 180*24*time.Hour, false},
 		// Rejected inputs.
 		{"6", 0, true},   // bare number, no unit
+		{"0", 0, true},   // bare number; time.ParseDuration accepts it
+		{"+0", 0, true},  // bare number; time.ParseDuration accepts it
 		{"5x", 0, true},  // unknown unit
 		{"-5d", 0, true}, // negative, extended unit
 		{"-5h", 0, true}, // negative, Go unit