Task spec API is ambiguous; propose canonical <dataset>:<format>:<metric> schema

## Problem
Task specs are currently composable colon-delimited variants (`task[:variant1[:variant2...]]`), but the semantics of each segment are not explicit or enforced.

Concrete example from current quickstart docs:
- `humaneval:3shot` conveys prompt format / few-shot regime, but not metric.
- `humaneval:bpb` conveys metric, but not format/regime.

## Proposal
Adopt an explicit minimal schema for task specs and apply it consistently. For example:

`::`

Examples:
- `humaneval:3shot:bpb`
- `humaneval:0shot:pass_at_1`

There's an open question, of course, for what `mc|rc` count as vs `0shot|3shot|8shot` count as