fix: restrict BEGIN_COMMIT_OVERRIDE to PR authors with write access

[evilgensec]

Summary

BEGIN_COMMIT_OVERRIDE in a merged PR's body lets the body's text replace the merge commit message that drives release notes, version bumps, and CHANGELOG generation. The PR body is fetched live at release-please run time (src/github.ts GraphQL query on pullRequest.body), and GitHub keeps the PR body editable by its author indefinitely after merge. An external contributor whose otherwise innocuous PR has been merged can later rewrite their PR body to insert a forged commit message — for example, BEGIN_COMMIT_OVERRIDE\nfeat!: x\n\nBREAKING CHANGE: <phishing markdown>\nEND_COMMIT_OVERRIDE — and that text drives the next CHANGELOG entry, forces a major version bump, and ends up in the published GitHub Release notes without any maintainer review.

This PR gates the override on the PR's authorAssociation (already exposed by the GitHub GraphQL PullRequest type) and honors it only for OWNER, MEMBER, and COLLABORATOR — the roles that already have write access to the repository. PRs from CONTRIBUTOR / FIRST_TIME_CONTRIBUTOR / NONE fall back to the merge commit message, matching the pre-BEGIN_COMMIT_OVERRIDE behavior.

#1161 added BEGIN_COMMIT_OVERRIDE for the maintainer "retroactively fix a release note" workflow; the README still describes it that way. The check here keeps the original maintainer workflow intact (maintainers are OWNER/MEMBER/COLLABORATOR) while closing the path that lets non-maintainer authors abuse it.

Changes

• src/pull-request.ts: add PullRequestAuthorAssociation type and optional authorAssociation field on PullRequest.
• src/github.ts: request authorAssociation in the mergeCommitsGraphQL query and thread it onto commit.pullRequest.
• src/commit.ts: in preprocessCommitMessage, ignore BEGIN_COMMIT_OVERRIDE unless authorAssociation is OWNER, MEMBER, or COLLABORATOR.
• test/commits.ts: existing override tests now set authorAssociation to a trusted value; two new tests cover (a) CONTRIBUTOR override is ignored and (b) missing authorAssociation is treated as untrusted.
• test/manifest.ts: existing manifest-mode override fixture sets authorAssociation: 'MEMBER'.

Test plan

• npm run lint — no new warnings or errors.
• npm test — 1101 passing, 0 failing.
• New tests cover both the trust path (MEMBER/OWNER still honored) and the deny paths (CONTRIBUTOR and missing authorAssociation fall back to the commit message).