cleanup-ephemeral-docs workflow: auto-merge disabled at repo level + duplicate PRs per merge

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## Summary

The `Cleanup ephemeral docs` workflow (`.github/workflows/cleanup-ephemeral-docs.yml`) successfully detects `PLAN.md`/`HANDOVER.md` on `master` and creates cleanup PRs, but those PRs accumulate unmerged because:

1. **Auto-merge is disabled at the repository level.** The workflow calls `gh pr merge --auto --squash --delete-branch`, which fails with `GraphQL: Auto merge is not allowed for this repository (enablePullRequestAutoMerge)`. The workflow logs the failure and continues, leaving the cleanup PR open for manual merge — which is rarely done.
2. **The workflow fires twice on every squash merge** — once for `pull_request_target.closed`, once for `push` to `master` — producing two near-identical cleanup PRs per upstream merge.

## Evidence

**Auto-merge failure** — from the run that fired after PR #7252 merged:

```
gh pr merge "$PR_URL" --auto --squash --delete-branch || echo "Auto-merge not available, PR requires manual merge"
...
GraphQL: Auto merge is not allowed for this repository (enablePullRequestAutoMerge)
Auto-merge not available, PR requires manual merge
Cleanup PR created and auto-merge enabled
```

(The trailing "auto-merge enabled" log line is misleading — `||` swallows the failure.)

**Run:** https://github.com/influxdata/docs-v2/actions/runs/26246670604

**Currently-open cleanup PRs awaiting manual merge:**

| PR | Created | Upstream trigger |
|---|---|---|
| #7255 | 2026-05-21 18:56 UTC | merge of #7252 (push event) |
| #7254 | 2026-05-21 18:56 UTC | merge of #7252 (pull_request_target event) |
| #7226 | 2026-05-15 17:02 UTC | unknown upstream merge |
| #7224 | 2026-05-15 16:04 UTC | unknown upstream merge |
| #7223 | 2026-05-15 15:53 UTC | unknown upstream merge |

Five open cleanup PRs across two days, each holding back a one-line deletion. `PLAN.md` is still on `master` from the most recent merge.

## Fix options

**For the auto-merge failure:**

- **Option A — Enable auto-merge in repo settings** (Settings → General → Pull Requests → "Allow auto-merge"). One-time fix; the existing workflow then works as designed. Lowest blast radius.
- **Option B — Switch to immediate squash merge with admin permission.** Replace `--auto` with `--admin --squash`. Requires the workflow's `GITHUB_TOKEN` (or a PAT) to have permission to bypass branch protection. Higher blast radius, but no manual setting toggle needed.
- **Option C — Skip the cleanup PR entirely; push the deletion commit directly.** Requires either disabling branch protection for the workflow's bot identity, or a separate admin token. Same trust trade-off as Option B.

Option A is the recommended path.

**For the duplicate-PR problem:**

- Drop the `push` trigger and rely only on `pull_request_target.closed`. Squash merges produce both events, so picking one is sufficient; `pull_request_target` is the more reliable source (it has access to the source-PR number for the preservation comment).
- Keep the `push` trigger as a backstop only for direct pushes to `master` — but gate on `github.event.head_commit.author.email != github.actions[bot]@users.noreply.github.com` to avoid feedback loops.

## Cleanup tasks (separate from this fix)

- Close #7254, #7255, #7223, #7224, #7226 — keep the latest one and merge it, or close all five and let the next merge create a fresh one once auto-merge is fixed.
- Audit whether the `Preserve plan details on source PR` comment fired correctly on all five (it should have, but the comment uses `gh pr comment` against `${{ github.event.pull_request.number }}` which only exists in the `pull_request_target` event).

## Acceptance

- The next merge to `master` that includes `PLAN.md` or `HANDOVER.md` produces exactly **one** cleanup PR that auto-merges within the configured CI window.
- The five currently-open cleanup PRs are closed or merged.

Start with .github/workflows/cleanup-ephemeral-docs.yml and inspect its pull_request_target.closed and push triggers, merge command, and logging. Check the repository's pull-request settings and the open cleanup PRs listed in the issue. Done means the next qualifying merge creates exactly one cleanup PR that auto-merges, and the existing five PRs are closed or merged.

Written by the indexing model from the issue text.