Migrate create-github-app-token from app-id to client-id

[lex57ukr]

Goal

Silence the deprecation warning emitted by actions/create-github-app-token in
.github/workflows/publish.yml by migrating from the deprecated app-id input
to the new client-id input introduced in v3.1.0 (2026-04-11).

Scope

• Retrieve the GitHub App's Client ID (string value, e.g. Iv23li...) — distinct from the numeric App ID currently stored as APP_ID.
• Store the Client ID in the repo (as a variable per upstream convention, or as a secret) and update .github/workflows/publish.yml to pass client-id: instead of app-id:.
• Remove the now-unused APP_ID secret once the workflow is green.

Outcome

Publish workflow runs without the Input 'app-id' has been deprecated warning
and stays aligned with the upstream action's recommended usage, so a future
major bump that removes app-id entirely won't break releases.

Notes

• Upstream PR: feat: add client-id input and deprecate app-id actions/create-github-app-token#353 — app-id still works as a fallback today, so this is not urgent, but the input will be removed in a future major.
• Client ID is a public identifier; upstream README now uses vars.GITHUB_APP_CLIENT_ID rather than a secret.
• Workflow is currently pinned at v3.1.1 (SHA 1b10c78c7865c340bc4f6099eb2f838309f1e8c3); no version bump needed for this migration.