fix(skill-drift): remove add_reviewer step that fails on scheduled runs

[HazAT]

Why

The most recent SDK Skill Drift Detector run failed:
run 25321035855.

The agent itself worked fine — it created PR #111 and 4 issues (#112–#115). What turned the job red was the add_reviewer safe output:

add_reviewer: No pull_request_number provided and not in pull request context

The workflow runs on schedule: (cron), so there is no PR context, and the agent has no way to attach add_reviewer to the PR it just created in the same job — add_reviewer requires either a PR-event trigger or an explicit pull_request_number, neither of which is available here.

What changes

• Drop add-reviewer: from safe-outputs: in .github/workflows/skill-drift-check.md.
• Remove Step 4a item 5 (the instruction that told the agent to call add_reviewer).
• Add a short note pointing the agent at the dedicated reviewer-assignment workflow so it doesn't try to assign reviewers itself.
• Recompile skill-drift-check.lock.yml with gh-aw v0.71.1 (was v0.67.1) so CI picks up the latest compiler, pinned action SHAs, and bug fixes (notably protected-files object-form, pull_request step ordering, incremental max_patch_size measurement).

Why removing this is safe

Reviewer assignment is not lost: .github/workflows/skill-drift-assign-reviewers.yml already exists and maps each changed skills/sentry-*-sdk/** directory to its specific team owner — strictly more granular than the agent's union-of-teams approach.

Heads-up — separate issue not fixed here

The assign-reviewers workflow does not currently fire for these auto-created PRs, because gh-aw opens PRs using the default GITHUB_TOKEN, which by design does not trigger downstream pull_request events (GitHub's loop-prevention rule). Confirmed by gh run list --branch fix/skill-drift-2026-05-04-affc010fce0c14ce returning empty, and by the assign-reviewers workflow's lifetime history showing only 2 skipped runs.

That means today's effective behavior is "no reviewers assigned at all" on skill-drift PRs. To fix that properly we'd need either:

1. Configure gh-aw's create-pull-request to use a PAT/app token so pull_request events actually fire, or
2. Switch skill-drift-assign-reviewers.yml to a workflow_run trigger of skill-drift-check.lock.yml and have it enumerate open skill-drift-labeled PRs.

Filing this separately keeps the PR scoped to "stop the job from going red".

Verification

• grep -c "add_reviewer\|add-reviewer" .github/workflows/skill-drift-check.lock.yml → 0
• Lock file metadata: compiler_version: v0.71.1
• gh aw compile → 1 workflow(s): 0 error(s), 0 warning(s)