feat(skill): support manual package release for Casbin adapter repos

[BeiBaiNian]

Summary

Related: apache/casbin#1747

As requested in apache/casbin#1747,
Casbin adapter repositories have removed semantic-release and stopped
auto-publishing to package registries. This PR extends the
incubator-release-assistant skill so a code agent can guide the manual
version release for those repositories (crates.io / Maven Central) using
prompts, without changing the Go engine or the config schema.

Changes:

• skills/incubator-release-assistant/SKILL.md:
  • update the skill description to cover manual package releases;
  • add a ## Publish adapter packages manually section with a six-step
    workflow: read the reference, verify tag/version/CI readiness, wait for
    explicit user authorization, publish with credentials from environment
    variables or external secret storage, verify the registry, and never
    guess versions or re-run a succeeded publish;
  • add three boundaries: registry credentials never enter the repository,
    publishing is irreversible and always requires human authorization, and
    version numbers/tags are never auto-created.
• skills/incubator-release-assistant/references/manual-package-release.md
  (new): documents the supported repositories (casbin-sqlx-adapter →
  cargo publish to crates.io, casbin-jcasbin-jdbc-adapter → mvn deploy
  to Maven Central), pre-flight checks, PowerShell/Bash commands, verification
  URLs, failure/rollback guidance (cargo yank), and the agent response
  format.

No Go code, config schema, or existing RC workflow is changed.

Verification

• the new reference follows the style of the existing
  references/prerequisites.md and references/configuration.md;
• the new SKILL section matches the tone and structure of the existing
  sections (## Prepare in the untrusted-code domain, ## Sign in the trusted domain, etc.);
• git diff --check passes;
• the existing RC workflow (doctor / validate / plan / prepare / sign /
  stage / verify-public) is untouched.

[CLAassistant]

All committers have signed the CLA.