ci(label-pr): run labeler with target context

[Will-hxw]

Background

Fork PRs can trigger the current Label PR workflow with a read-only GITHUB_TOKEN. The label step then fails while calling gh pr edit --add-label, for example in #1097 with GraphQL: Resource not accessible by integration (addLabelsToLabelable).

What Changed

• Run the label workflow on pull_request_target so it executes in the base repository context and can update PR labels for fork PRs.
• Keep explicit least-privilege permissions, adding contents: read alongside the existing label-related write scopes.
• Remove the checkout step because this workflow only reads PR metadata and edits labels.
• Set GH_REPO explicitly so gh pr view/edit works without a checked-out repository.

Not Changed

• The conventional commit prefix to label mapping is unchanged.
• No project code, tests, dependencies, or build configuration are changed.

Risk

Low for this workflow because it does not checkout or execute pull request code. It only reads the PR title/number from the event payload and uses gh to update labels.

Validation

• Parsed .github/workflows/label-pr.yml with the repository yaml package.
• pnpm exec prettier -c .github/workflows/label-pr.yml
• git diff --check

Linked Issues

Unblocks fork PR labeling failures such as #1097.