Unpinned floating tags (e.g. `v4`) on immutable Actions are not flagged by `actions/unpinned-tag`

閱讀 actions/ql/src/Security/CWE-829/UnpinnedActionsTag.ql 和 actions/ql/lib/codeql/actions/security/UseOfUnversionedImmutableAction.qll，從 isImmutableAction 和現有的 SemVer predicate 開始。完成的標準是：完整的 vX.Y.Z 標籤和完整的 SHA 仍維持豁免，而 vX 和 vX.Y 等浮動參照會被回報，並包含所需的變更說明。

Description of the issue

The actions/unpinned-tag query (actions/ql/src/Security/CWE-829/UnpinnedActionsTag.ql) exempts any Action on the immutable-actions allow list from the unpinned-tag warning, regardless of the ref used. The exclusion is version-independent:

not exists(UsesStep step | uses = step and isImmutableAction(step, nwo))

and isImmutableAction (actions/ql/lib/codeql/actions/security/UseOfUnversionedImmutableAction.qll) only checks membership in immutableActionsDataModel(nwo); it never inspects the version.

Why this is a gap

GitHub's immutability guarantee only applies to fully-expanded SemVer release tags (vX.Y.Z) and full commit SHAs. Floating tags such as v4, v4.0 and main remain mutable: maintainers move them to the latest matching release, so they can change under a consumer exactly like any other tag. See Using immutable releases and tags to manage your action's releases.

As a result, a reference like actions/checkout@v2 is flagged by neither query:

• UnpinnedActionsTag skips it because actions/checkout is on the immutable list.
• UnversionedImmutableAction skips it because its isSemVer predicate accepts a bare major tag like v2.

So a genuinely mutable floating tag on an immutable Action goes unwarned.

Suggested direction

Narrow the exemption so an immutable Action is only exempt when pinned to a full vX.Y.Z (or a SHA), for example:

not (isImmutableAction(step, nwo) and isFullSemVer(version))

with an isFullSemVer stricter than the current isSemVer (which also matches floating vX and vX.Y). This would need care because the immutable-action model is shared with the experimental UnversionedImmutableAction query, and it would increase alert volume for consumers pinning immutable Actions to floating major tags, so it deserves its own change note and review.

Filed as a follow-up to #22409 (which is scoped to the trusted-owner allow list and does not address this).