feat: add IngestSpec value object (#59)

[underchemist]

Summary

Implements #59 — the first slice of the end-to-end ingest workflow (parent #56).

Adds IngestSpec, a parsed, immutable ingestion specification that is the typed entry point for building an ingestion from Python, and teaches IngestResource.create to accept it.

What's included

• prescient_sdk/ingest_spec.py — IngestSpec:
  • from_file(path) / from_dict(spec) parse the spec (via yaml.safe_load).
  • .spec returns a deep copy (immutable); .to_bytes() serializes back to YAML with key order preserved.
  • local_locations() reports locations whose path is not an s3:// URI
• IngestResource.create now accepts Path | str | bytes | IngestSpec. For an IngestSpec it raises ValueError (naming the offending locations) if any location is still local; otherwise it submits .to_bytes(). Raw bytes/str/Path inputs are unchanged.
• pyyaml promoted to a runtime dependency; IngestSpec exported from prescient_sdk.

Staging (uploading local sources + rewriting their paths) is intentionally out of scope here — that's #61 (blocked by this and #60).

Acceptance criteria

• IngestSpec.from_file / from_dict parse a spec; .to_bytes() round-trips
• IngestResource.create accepts an all-s3:// IngestSpec and submits the serialized spec
• create raises ValueError naming the offending location when an IngestSpec has a non-s3:// location path
• pyyaml declared as a runtime dependency; IngestSpec exported from prescient_sdk
• Unit tests cover parse, round-trip, create-with-IngestSpec, and the guardrail

Testing

• uv run pytest — 124 passed
• uv run ruff check prescient_sdk/ tests/ — clean; files formatted with ruff format

Closes #59

🤖 Generated with Claude Code

[michaelconnor00]

Reviewing this is bringing up a question: Should the IngestResource.create() take a file path (or bytes), or should it only take an IngestSpec, which will read from file, or take bytes??

I had also envisioned the IngestSpec would have a Pydantic model for validating the spec, which then implies the answer to the above is IngestResource.create() should only take IngestSpec and an argument.

[michaelconnor00]

I think it makes sense to create a seperate issue for adding the pydantic model and syncing it with the ingest API.

However for this PR, we should change IngestResource.create() to only take IngestSepc. Unless you have thoughts otherwise.

[underchemist]

Sure that sounds good