fix: ignore inherited snapshot usage vars

[brownie-ricon]

Summary

• parse wp snapshot flags from argv instead of inherited usage_* env vars
• add regression coverage for parent mise-session leakage
• sort README task rows deterministically and regenerate README

Verification

• mise run test
• bash -n .mise/tasks/snapshot test/snapshot.bats
• shellcheck -x .mise/tasks/snapshot
• codebase lint:mise-settings "$PWD"
• git diff --check