fix(pr-skill): add no-changelog label step for non-consumer changes

When a PR has no consumer-facing changes, the skill now adds the
`no-changelog` label instead of silently skipping. This prevents
CI changelog validation failures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sirtimid

.claude/skills/pr/SKILL.md

@@ -123,7 +123,10 @@ Otherwise, after all launched subagents complete:
 
 ## Phase 5: Update changelogs
 
-**MANDATORY — DO NOT SKIP.** Each PR must update changelogs for all packages with consumer-facing changes.
+**MANDATORY — DO NOT SKIP.** Analyze the diff and determine whether any changes are consumer-facing (i.e., affect the behavior or API of a published or private package).
+
+- **If there are NO consumer-facing changes** (e.g., docs-only, CI, tooling, skill definitions, dev scripts): add the `no-changelog` label to the PR via `gh pr edit <number> --add-label no-changelog` and skip the rest of this phase.
+- **If there ARE consumer-facing changes**: update changelogs as described below.
 
 Read the instructions in [`docs/contributing/updating-changelogs.md`](../../../docs/contributing/updating-changelogs.md) and follow them **to the letter**. In particular: