DOC Correct YAML objective tag in dataset overview

[Chen Yufeiyang (feiiiiii5)]

Description

The dataset overview still recommends is_objective: true, which the current YAML loader rejects as an extra field. Use seed_type: objective and describe SeedDataset.objectives as a filtered property over seeds.

Tests and Documentation

Using Python 3.13.3, dependencies pinned to uv.lock, and the actual checkout's SeedDataset.from_yaml_file:

• The old flag raises extra_forbidden at seeds.0.prompt.is_objective.
• seed_type: objective loads a SeedObjective both per seed and as a dataset default. It retains local origin and is the same object in seeds and objectives.
• git diff --check passes.

Only this Markdown paragraph changes. The full test suite, pre-commit suite, documentation build, and JupyText were not run.