fix(review): accept language-only SDK classifications #5515

User Story

As an OpenShell maintainer migrating SDK classifications to language-specific topics, I need the label validator to accept those topics without the removed topic:sdk label.

Description

#5503 removes the broad SDK topic, but label_issue_policy.py requires it whenever a language topic is selected. Remove that requirement, update its regression test to use the language-only taxonomy, and remove the matching instruction from the labeling skill.

For example, a supported topic:sdk:python classification must validate without topic:sdk. Existing evidence and active-label checks remain in place.

Part of #5151; do not create labels or migrate item assignments in this fix.

Definition of Done

• Language-specific SDK topics validate against a taxonomy containing no broad SDK topic.
• The skill matches the language-only taxonomy.
• Focused policy tests pass.