docs(action.yml): mirror_verbose description promises '(0-3)' but validation only checks >= 0

gh issue create --title "docs(action.yml): mirror_verbose description promises '(0-3)' but validation only checks >= 0" --label "documentation,audit,priority:low" --body "## Finding

The mirror_verbose input description in action.yml says '(0-3)'. The validator in lib.sh only checks >= 0. A value of 5 passes validation; lftp's -v flag does not have an upper bound, but the documented contract says 0-3.

## Affected code

- action.yml — mirror_verbose description
- lib.sh — mirror_verbose validator

## Reproduction

1. Set INPUT_MIRROR_VERBOSE=5.
2. Validation passes; lftp runs with -v 5.

## Suggested fix

Either:
- Loosen the action.yml description to remove the upper bound ('Verbose level for lftp mirror (>= 0). Higher = noisier.').
- Or tighten the validator to reject > 3.

The first option is safer; lftp tolerates any level.

## Source

F2 audit 2026-09-03 (action.yml audit). Reporter: subagent-pekodbj7.

## Related

- Part of EPIC #[epic-number]
- Closes: #NONE (no existing issue)"

Compare the mirror_verbose description in action.yml with its validator in lib.sh, then review the reproduction using INPUT_MIRROR_VERBOSE=5. Resolve the documented range consistently, favoring the safer documentation change noted in the issue, and verify that the resulting contract matches lftp behavior and validation.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.