Declare the error contract, camelCase params, and cursor pagination

[ExtraToast]

What to build

knowledge joins the estate contract conventions: declared errors, camelCase parameters, and cursor pagination on its genuinely unbounded collections.

This service is the furthest from the house standard — 18 operations declaring only 200, snake_case query parameters (target_id, stale_days, low_confidence_max) against camelCase everywhere else, and a bare limit on 11 operations with no cursor.

Note that the parameter rename is a breaking contract change and will take its clients to a major version.

Acceptance criteria

• Error responses declared as application/problem+json
• Query and path parameters are camelCase
• Note listings paginate using the estate cursor convention
• The breaking rename is landed deliberately, with consumers updated in step

Blocked by

• Declare ProblemDetail responses on every operation (kotlin-spring-commons tracker)