Optional usage-based navigation hints for agents

OKF provides the structure of the knowledge catalog, but an agent starting from an index has no information about which concepts have historically been useful for a particular intent or set of conditions.

No files, tests, or entry points are named. Start by locating the knowledge-catalog index structure and the agent path that reads it, then clarify how intent, conditions, and optional usage-based hints should be represented; done should include an agreed design and coverage showing existing indexes still work.

Written by the indexing model from the issue text.