fix(ci): skip pull request automation without write credentials

[marandaneto]

Problem

Some pull request workflows try to write comments or update project boards. GitHub does not provide the required write token or GitHub App credentials to fork and Dependabot pull requests.

The changeset hygiene workflow completes its setup and report generation before its comment request fails with a 403 response. Example failure: https://github.com/PostHog/posthog-js/actions/runs/32717251565/job/97401261439?pr=4614

The feature flags project-board workflow already skips some restricted runs. Its guard only checks fork status for pull_request, not pull_request_review. It also checks github.actor for Dependabot, but that value identifies the reviewer during review events.

Changes

• Skip changeset hygiene for fork and Dependabot pull requests before a runner is provisioned.
• Skip feature flags project-board updates for fork and Dependabot pull requests on both pull request and review events.
• Continue running project-board updates for manual dispatches and same-repository pull requests.

GitHub treats skipped jobs as successful.

Testing

• actionlint .github/workflows/changeset-hygiene.yml .github/workflows/flags-project-board.yml
• Verified the project-board condition for manual, same-repository, fork, and Dependabot events.
• git diff --check origin/main...HEAD