Add labels to BaseArtifact

[quietflareco-sudo]

Summary

Closes #181.

labels: dict[str, str] = Field(default_factory=dict)

outputs:
  - id: scored
    kind: file
    path: batch_017.parquet
    labels: {subject: batch_017, role: measurement}

The runtime never interprets them. They travel with the artifact through serialization and exist for whatever reads a workflow afterwards: a UI grouping nodes, or a lineage reader answering "everything derived from subject X". A domain expresses its own structure through labels instead of the runtime growing a field per domain.

Docstring shortened per review.

Not a cache concern

HorusTask._fingerprint hashes artifact content digests plus a hash of the runtime and executor models. Artifact models are not in it, so labels cannot reach a cache decision. Relabelling describes an artifact differently, it does not make its contents stale.

Why str values

Strings stay serializable in YAML and JSON without surprises and can be used directly as index keys. Easy to widen to dict[str, Any] later, hard to narrow. Happy to change it.

Tests

Four: defaults to empty, not shared between instances (a mutable default would leak one artifact's labels into every other), survives a model_dump / model_validate round trip, rejects non-string values.

737 passed, coverage 91.29%, make lint clean.

Documentation impact

If a user, plugin, or GUI can observe the difference, it requires documentation.

• No documentation update required
• Documentation updated / will be updated

horus-docs PR: to follow.

This is a new field workflow authors write in YAML, so it belongs in sdk/core/artifact.mdx and guides/writing-workflows-yaml.mdx. Happy to write it, or leave it to you.

[quietflareco-sudo]

@chdominguez FYI! Could you please review this? Thanks!

[chdominguez]

Small change on the docstring and ready to go!

[chdominguez]

lgtm

[chdominguez]

Docs PR: temple-compute/horus-docs#58