Remove actionlint self-repository syntax workaround after upstream support ships

[robzolkos]

Context

PR #210 adopts GitHub's $/.github/... self-repository syntax for reusable workflow calls. This syntax resolves workflow references from the exact commit being run.

Actionlint v1.7.12 does not yet recognize $/.github/..., so .github/actionlint.yaml narrowly suppresses its workflow-call false positive for these references. Zizmor remains fully enabled.

Upstream tracking:

• actionlint issue: Support the new $/ self-repository uses: syntax rhysd/actionlint#711
• actionlint implementation PR: Support the $/ self-repository uses: syntax rhysd/actionlint#732

Done when

• A released actionlint version supports $/ self-repository references.
• The pinned rhysd/actionlint action is updated to that release.
• The corresponding suppression is removed from .github/actionlint.yaml (and the file is removed if empty).
• Actionlint, zizmor, and the full CI suite pass without the suppression.