Remove actionlint self-repository syntax workaround after upstream support ships

## 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: https://github.com/rhysd/actionlint/issues/711
- actionlint implementation PR: https://github.com/rhysd/actionlint/pull/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.

Check whether a released actionlint version supports `$/` self-repository references, then inspect the pinned `rhysd/actionlint` action and `.github/actionlint.yaml`. Update the pin and remove the corresponding suppression, deleting the file if empty. Run actionlint, zizmor, and the full CI suite to confirm they pass without the workaround.

Written by the indexing model from the issue text.