## Description

Switch the `openapi-lint` CI job from upstream `@stoplight/spectral-cli`
+ pinned `@stoplight/spectral-rulesets` to the Camunda fork
[`@camunda8/spectral-cli`](https://www.npmjs.com/package/@camunda8/spectral-cli).

The fork is a single pre-bundled package (no transitive dependency on
`@stoplight/spectral-rulesets`) that fixes three upstream issues:

1. **nimma null-deref** — the `duplicated-entry-in-enum` rule crashes on
`null` example values. The fork adds null guards to the JSONPath filter.
2. **Unicode regex false positives** — Ajv rejects valid Unicode
patterns (e.g. `\p{L}`) because `unicodeRegExp` defaults to `false`. The
fork integrates [PR
camunda#2809](stoplightio/spectral#2809) and defaults
`unicodeRegExp` to `true`.
3. **`ignoreUnknownFormat` at ruleset level** — allows rulesets to
suppress `unrecognized-format` warnings without requiring CLI flags.

Fork repo: https://github.com/camunda/camunda-spectral

## Changes

### CI workflow (`.github/workflows/ci.yml`)
- Replace `@stoplight/spectral-cli@6.16.0` +
`@stoplight/spectral-rulesets@1.22.2` with
`@camunda8/spectral-cli@6.16.1`
- Simplify install step to a single `npm install -g` call
- Remove the `spectral-rulesets` pinning workaround comment

### Spectral ruleset (`.spectral.yaml`)
- Add `ignoreUnknownFormat: true` to suppress `unrecognized-format`
warnings on YAML partials (eliminates 43 warnings on the file-level
pass)
- Promote `oas3-valid-media-example` from `warn` to `error` (all media
example issues are now fixed)
- Keep `oas3-valid-schema-example` at `warn` — ScopeKey uses `oneOf`
with structurally identical branches (`ProcessInstanceKey`,
`ElementInstanceKey`), causing "must match exactly one schema in oneOf"
on any ScopeKey example. Changing `oneOf` to `anyOf` would fix the
warning but breaks Java code generation. 4 warnings remain from this.
- Remove the `TODO` comment about restoring severity (no longer needed
for media examples)

### OpenAPI spec fixes
- `decision-definitions.yaml`: Fix example `decisionDefinitionId` from
`1234-5678` (starts with digit, violates `^[\p{L}_]...` pattern) to
`my-decision`
- `system.yaml`: Add `type: string` to nullable `stage` property
(Spectral requires `type` alongside `nullable`), change example from
`null` to `"prod"`

### What was NOT changed (and why)
- `keys.yaml` ScopeKey `oneOf` — left as-is because changing to `anyOf`
alters the generated Java classes and breaks the build (confirmed by CI
failure on earlier push)
- `process-instances.yaml` `ancestorElementInstanceKey` `oneOf` — same
reason; changing to `allOf` breaks codegen

Closes camunda#54071