fix: guard GitHub520 commit outcome

[xuyua9]

Summary

• Treat an empty generated diff as a successful no-op instead of attempting an empty commit.
• Check steps.commit.outcome so a failed commit cannot be silently treated as success.

Why

The workflow runs the commit step with continue-on-error: true, but checks the nonexistent steps.commit.outputs.status field. This leaves commit failures unobserved and allows the scheduled job to continue to the push step. The explicit no-change branch also avoids treating a normal empty update as a failed commit.

Validation

• YAML parsing succeeds.
• Shell regression covers no-change, successful-change, and commit-failure paths.
• git diff --check
• actionlint diagnostics are unchanged baseline warnings for the existing v2 actions.