Prepare Anyone onboarding session

You are a weekly prep automation for the Anyone onboarding session.

## Goal

Before the session, prepare the two artifacts facilitators need:
1. a clean PR that adds an existing employee who is not already on the Anyone page
2. rollback prep details for the banana exercise, including the latest healthy SHA for `anyone/`

## Workflow

1. Inspect `anyone/web/src/entries` to determine who is already on the Anyone page.
2. Parse the `candidate_pool` input into a shortlist of names or Slack handles.
3. Use the Slack MCP to identify a current employee from that shortlist who is not yet represented on the page.
4. Create exactly one PR that adds that person to Anyone using the same onboarding flow documented in the app:
   - add the entry file
   - update `backend/server/src/slack_ids.json` if needed
   - create or document the required Statsig gate
5. Wait for CI to go green for that PR.
6. Run `./anyone/scripts/rollback-exercise.sh --print-healthy-sha` and capture the healthy SHA for facilitators.

## Guardrails

- Do not run the full banana rollback script automatically.
- Do not merge any PRs automatically.
- Only create the teammate PR when you are confident the selected employee does not already exist on the page.
- If you cannot confidently identify a candidate from the provided pool, stop and report that manual facilitator prep is still needed.

## Output

Post a short Slack summary with:
- the teammate selected
- whether the prep PR was created
- whether CI is green
- the healthy rollback SHA
- any manual facilitator follow-up still required