Public-repository organization gate is bypassed for push webhooks

Summary

When PUBLIC_REPO_ORG_GATE is configured, the GitHub push webhook path bypasses the public-repository organization membership check.

Code reference

• agent/webhooks/github_routes.py:83-88
• agent/webhooks/common.py:630-650

The push branch checks only _is_repo_auto_review_enabled() and schedules process_github_push_event(). It returns before calling _enforce_public_repo_org_gate().

Reproduction

1. Configure PUBLIC_REPO_ORG_GATE=trusted-org.
2. Enable automatic reviews for a public repository.
3. Send a valid signed push payload for that repository with sender.login set to a user who is not an active member of trusted-org.
4. Observe that the endpoint returns status=accepted and schedules reviewer-watch evaluation.

Equivalent pull_request, issue, and comment paths invoke the gate and reject the same sender.

Expected behavior

All agent-triggering public-repository webhook paths, including push, enforce PUBLIC_REPO_ORG_GATE consistently.

Actual behavior

A non-member can trigger the push-driven reviewer evaluation for an opted-in public repository.

Why it matters

This bypasses an explicitly configured access-control policy and can cause unauthorized reviewer runs, GitHub API usage, and review comments.

Suggested fix

Call _enforce_public_repo_org_gate(payload, event_type) before scheduling process_github_push_event, and add accepted/rejected route tests for public, private, member, and non-member senders.