docs(integrations): document event catalog custom events

[karlrank]

What

Adds Custom Events to the Integrations section — the first narrative documentation for the Event Catalog's custom event feature.

Until now the only coverage was the auto-generated API reference at /api/event-catalog. It renders the live spec, so the five custom-event operations appear there, but nothing explained the lifecycle, the projection model, or how a custom event reaches webhooks and automations. A repo-wide search for "custom event" returned no related hits.

The new page

docs/integrations/custom-events.md (sidebar position 4, right after Core Events):

• Why custom events exist — a built-in cannot project attributes that only one organization has
• Anatomy — root entity, entity graph, graph_source projection, guided vs JSONata mapping, the three trigger sources, and the assembled payload with a full example
• The five-step builder in epilot 360
• Lifecycle — draft → active → deprecated, with a state diagram and an explicit table of what is mutable when
• Deriving from a built-in — lineage, the restrictions that are inherited and cannot be loosened, and the base auto-trigger choice at publish time
• Consuming — webhooks (event_<EventName>), automations in both directions, schema and history endpoints
• API walkthrough — create → preview → publish → trigger → deprecate, with curl
• Reference — naming rules, limits, reserved field names, what the API validates, and the current v1 limitations

Content is sourced from the published OpenAPI spec, the event-catalog-api handlers and validators, and the epilot360-event-catalog builder — not from the original RFC, which predates several shipped decisions.

Also in this MR

• docs/sdk/clients/event-catalog.md regenerated. It predated custom events and listed 7 of the API's 15 operations. Now includes createCustomEvent, replaceCustomEventDraft, previewCustomEvent, publishCustomEventDefinition, deprecateCustomEvent, plus listEventVersions, searchEventHistoryV2 and getHistoricalEvent.
• Cross-link from Core Events, a short Business Events section in the Integrations overview, and a sidebar renumber so the new page slots in cleanly.

Not in this MR

The CLI reference (docs/cli/commands/event-catalog.md) is still stale — same 7 operations. Its upstream source, sdk-js/packages/cli/docs/, is itself out of date, so npm run update-cli cannot fix it from here. It needs pnpm generate in sdk-js/packages/cli first, which also regenerates that package's src/ and definitions/ — a separate MR in sdk-js.

Verification

• npm run build passes. The site sets onBrokenLinks: 'throw', so every link in the new page resolves, and the mermaid diagram renders.
• npm run lint has 24 pre-existing prettier errors in src/components/{AgentToolkitPromo,CollapsibleJsonViewer,LottieAnimation}.tsx, all on main and untouched here — this MR adds nothing under src/.

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