task: make the assistant's existing telemetry reportable (GA4 custom dimensions, gtag after GTM swap, unanswered-question review)

The docs assistant already emits GA4 events and already logs questions server-side. Neither is reportable yet, for two separate reasons. Both are configuration, not code.

## What already exists

**Client**, `widget.js` in `celo-org/docs-ai-assistant` — seven events through the page's existing `window.gtag`:

| Event | Parameters |
|---|---|
| `assistant_question` | `answered`, `escalated`, `truncated` |
| `assistant_error` | `from_api`, `status` |
| `assistant_citation_click` | `href` |
| `assistant_opened`, `assistant_escalate`, `assistant_new_chat`, `assistant_copy` | — |

**Server**, `app/api/chat/route.ts` — `logQuestion()` pushes `{question, model, citedUrls, answered, timestamp, refused}` onto the Upstash Redis list `docs-assistant:questions`, trimmed to the most recent 10,000.

## Gap 1: the event parameters are not registered as custom dimensions

GA4 drops event parameters that are not registered. The events arrive; `answered`, `escalated`, `truncated`, `from_api`, `status` and `href` cannot be broken out in any report until they are added as **event-scoped custom dimensions** in the GA4 property for `G-0CXEKQ81V2`.

`answered` is the one that matters — it is the share of questions the assistant could not answer from the docs, which is the docs-gap rate.

## Gap 2: `track()` is a silent no-op without `window.gtag`

```js
function track(name, params) {
if (typeof window.gtag === 'function') window.gtag('event', name, params || {});
}
```

`window.gtag` is provided today by `integrations.ga4` in `docs.json`. #2300 replaces that with `integrations.gtm`. GTM's Google tag should still define the global, but this exact configuration has already produced one silent failure — the container shipped with its own ID in the Tag ID field, so it loaded, fired, errored nowhere, and sent nothing to GA4. Verify rather than assume.

## Gap 3: nothing reads the Redis list

The list is the actual docs-gap signal — the uncited questions in it are the pages that need writing — and there is no routine for reading it. A monthly pull of entries where `answered: false` would close the loop that #2250 used to justify building the assistant instead of paying for Mintlify Pro.

## Acceptance criteria

- [ ] The six assistant parameters registered as event-scoped custom dimensions in GA4
- [ ] After #2300 merges, `typeof window.gtag === 'function'` confirmed on a published page, and one `assistant_question` event seen in GA4 Realtime
- [ ] A stated cadence and owner for reviewing `docs-assistant:questions` where `answered: false`
- [ ] ANALYTICS.md records whichever of these lands

Refs #2250, #2286, #2300.

Review widget.js, app/api/chat/route.ts, docs.json, and the ANALYTICS.md requirements, then verify the post-#2300 published-page behavior in GA4 Realtime and inspect the docs-assistant:questions Redis list. Done means the six event parameters are registered, an assistant_question event is confirmed, unanswered questions have a stated review cadence and owner, and ANALYTICS.md records the results.

Written by the indexing model from the issue text.