feat: carry primary_task and label_window_days into WorldSpec for dataset card rendering

[shaypal5]

Context

Currently render_dataset_card() hard-codes converted_within_90_days and the 90-day label window as literals. This was flagged in PR #5 (Copilot comment: #5 (comment)).

Problem

Once additional recipes or configurable task windows are introduced, the dataset card will drift from the actual resolved spec.

Proposed solution

• Add primary_task: str and label_window_days: int fields to GenerationConfig (with defaults matching current behaviour)
• Propagate them through Recipe.from_dict() / resolve_config() and the recipe YAML schema
• render_dataset_card() reads from world_spec.config instead of literals

Scope

Deferred to M3 (schema layer) or wherever recipe YAML schema is formally versioned. Not a blocker for v0.2.0 — v1 has exactly one task and the 90-day window is an architecture invariant.