port_to_docs.js: auto-add --cluster-id for Enterprise in synced plugin install examples

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## What changed / what's needed

`helper-scripts/influxdb3-plugins/port_to_docs.js` transforms plugin READMEs from `influxdb3_plugins` into docs-v2 content, but doesn't account for Enterprise-only flags. The upstream `influxdb3 serve` install examples never include `--cluster-id` (correctly — that repo is edition-agnostic), so every synced page inherits that gap for InfluxDB 3 Enterprise.

Add a transform step, following the same pattern as the existing `schemaPlugins` insertion (`port_to_docs.js` lines ~260-298), that detects an `influxdb3 serve \` block containing `--node-id ... \` with no `--cluster-id`, and injects:

\`\`\`
{{< show-in "enterprise" >}}--cluster-id CLUSTER_ID \
{{< /show-in >}}--node-id NODE_ID \
\`\`\`

matching the pattern already used in `content/shared/influxdb3-admin/object-storage/{s3,gcs,minio,azure}.md`.

## Why

Found via docs-v2 #6878: `plugins-library/examples/wal-plugin.md` was missing `--cluster-id` in its Enterprise install example. Fixed by hand in #7720, but that page — and 13 others with the same `influxdb3 serve` install block (`basic-transformation.md`, `downsampler.md`, `notifier.md`, `system-metrics.md`, and more) — is regenerated by `.github/workflows/sync-plugins.yml` → `port_to_docs.js` on every plugin sync. A manual fix gets silently overwritten on the next sync.

## Impact

Without this, every future plugin sync reintroduces the missing-flag gap on all affected pages, and someone has to notice and re-patch by hand each time.

## Verification

- Confirmed 14 files under `content/shared/influxdb3-plugins/` contain an `influxdb3 serve` install block, all lacking `--cluster-id` before the #7720 fix.
- Confirmed `port_to_docs.js` has no existing Enterprise/cluster-id handling (`grep -n "cluster-id\|show-in\|enterprise"` — no matches outside the schema-section insertion).

Start in helper-scripts/influxdb3-plugins/port_to_docs.js, especially the existing schemaPlugins insertion around lines 260-298, and compare its transform pattern with content/shared/influxdb3-admin/object-storage/{s3,gcs,minio,azure}.md. Verify the transform detects an influxdb3 serve block with --node-id but no --cluster-id and adds the Enterprise-only flag to the synced plugin examples.

Written by the indexing model from the issue text.