Auto-review should post a comment when no issues are found (silent clean pass)

[MG674]

Problem

When claude-code-action runs an automatic PR review (triggered by pull_request event with a prompt) and finds no issues, it exits silently — no PR comment, no inline comments, no indication that it ran and reviewed the code.

The action runs successfully (5+ minutes, exit code 0, "No buffered inline comments" in logs), but the PR author has no way to know whether:

1. The review ran and found nothing (clean pass), or
2. The review silently failed or didn't actually analyse the code

Expected behaviour

Post a brief PR comment confirming the review completed with no findings, e.g.:

✅ Claude Code reviewed this PR and found no issues.

This is what Gemini Code Assist does — when it has no findings, it explicitly says so. This is much more helpful than silence, because it gives the author confidence that the review actually ran.

Context

• Using anthropics/claude-code-action@v1 with prompt input (not @claude trigger)
• Trigger: pull_request: [opened, synchronize, ready_for_review, reopened]
• The @claude review comment-triggered path works correctly and posts detailed reviews
• The automatic path runs but produces no visible output on clean passes
• show_full_output: false (default), so there's no way to see what Claude thought

Workaround

None currently — we rely on the @claude review comment trigger for visible feedback and treat the auto-review as a silent safety net, which reduces its value.

Suggestion

Add an option (or make it default) to post a brief "no issues found" comment when the review completes without findings. Even a one-line sticky comment would suffice.