fix(ci): renovate rejects unknown keys, so the disable never took effect (#1082)

Closes #1082.

When renovate was disabled in #1080, the block explaining why was written as a _comment key. JSON has no comment syntax, and _comment is not a renovate option.

Renovate does not ignore unknown top-level keys — it treats the whole file as invalid, stops processing, and opens an "Action Required: Fix Renovate Configuration" issue:

Location: .github/renovate.json Error type: The renovate configuration file contains some invalid settings Message: Invalid configuration option: _comment

So the config meant to quietly disable renovate became a standing complaint instead. (enabled: false never applied either, since the file was rejected wholesale — the queue stayed quiet only because renovate halts on an invalid config, which is not the same thing as being off.)

description is the documented field for annotating a renovate config, and it accepts an array of strings. Same text, in a key the schema actually accepts.

Verified the file parses and carries exactly three top-level keys: $schema, enabled, description.

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