lint:bats-test-task should recognize test/_default namespace tasks

[brownie-ricon]

Problem

codebase lint:bats-test-task only recognizes the file-task form:

.mise/tasks/test

It does not recognize the namespace default-task form:

.mise/tasks/test/_default

Mise treats that _default file as the implementation for mise run test, and this shape is needed when a repo also wants nested test tasks such as mise run test:python.

Concrete case

In KnickKnackLabs/permissions#2, the repo has:

.mise/tasks/test/_default  # canonical BATS runner for `mise run test`
.mise/tasks/test/python    # Python unit tests, invoked as `mise run test:python`

mise run test works and BATS passes, but codebase lint:bats-test-task reports:

OK    permissions (no .mise/tasks/test — nothing to check)

That is misleading because there is a BATS test task, just in namespace _default form.

Expected behavior

The lint should inspect .mise/tasks/test/_default when .mise/tasks/test is a directory and apply the same canonical BATS task checks to that file.

If both forms somehow exist or the directory has no _default, it should report an actionable result consistent with mise behavior.

Why it matters

Without this, repos that use conventional nested tasks such as test:python, test:unit, or test:integration lose the BATS task lint coverage while still appearing green.