Credit granularity and user contribution profile

[bdecost]

#9 adds granularity to Moment credits, grouping into data, ai, and infrastructure credit types

in the proposed user profiles, it would be nice to display a list of credit events that summarize / link to specific contributions. The simplest form of this could be a breakdown of credits across projects. For more specificity, we could augment the statistics.jsonl entries with a short description field, perhaps with optional reference to a specific issue or task.

Alternatively, in the proposed comment stream credit assignment mechanism in #19, credit is implicitly linked to a task through the issue the comment belongs to, so the user profile could link directly there. Users could assign credit in a pattern like <moment-credit-assignment-token> @user for [annotating 20 datapoints](mdf-link)

Individual contributions might have different granularity than a github issue -- some contributions might map cleanly to a pull request that resolves a specific issue, but data contributions in particular might have smaller granularity. If a project has an issue that specifies an annotation task where many users annotate portions of the data, there will be many credit assignments until the full dataset is annotated, at which point the issue will be closed