Use one duration parser and fix the --older-than months example (closes #123)

Two parseDuration functions existed with different grammars; only the one in internal/vaultik/helpers.go was reachable from a flag. The unused copy in internal/cli/duration.go is deleted, so no flag accepts anything it did not before.

The README gave 6m as the six-months example for snapshot purge --older-than, but m is minutes: that command removed every snapshot older than six minutes. The example is now 6mo, and the help for --older-than and --keep-newer-than states that m is minutes and mo is months.

The parser now rejects negative durations, which it used to accept or silently make positive.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (merge)

Co-authored-by: clawbot <clawbot@noreply.example.org>

README.md

@@ -251,7 +251,8 @@ local index alone, and still exits zero.
 per-snapshot-name (`--keep-latest` keeps the latest of each name, not the
 latest globally).
 * `--keep-latest`: Keep only the most recent snapshot of each name
-* `--older-than <duration>`: Remove snapshots older than duration (e.g. `30d`, `6m`, `1y`)
+* `--older-than <duration>`: Remove snapshots older than duration (e.g. `30d`,
+  `4w`, `6mo`, `1y`; `m` is minutes, `mo` is months)
 * `--snapshot <name>`: Restrict to specific snapshot names (repeat for multiple)
 * `--force`: Skip confirmation prompt