[P2] EPIC: Custom node + provider node + store node entry-point groups

[se-jo-ma]

Epic scope

Per demos/everything-demo/README.md:145 and harbor/TODO.md "High level" wishlist: no harbor.nodes entry-point group exists yet. Custom nodes use the module:ClassName IR kind: escape hatch.

Closing this gap means three coordinated additions:

Sub-items

• harbor.nodes entry-point group — pluggy group + register_nodes() hookspec + collect_nodes(pm) aggregator. Mirrors shipped harbor.mcp_adapters.
• harbor.providers entry-point group — provider nodes (LM, embedding, rerank, etc.).
• harbor.stores entry-point group — store node implementations distributable as pip packages.
• Deprecate module:ClassName escape hatch in favor of entry-point group registration (or document both paths).

References

• HARBOR_TODOS_REVIEW.md Part G + Part H.2
• harbor/TODO.md "High level — shipping the product"
• demos/everything-demo/README.md:145
• Existing pattern: harbor.mcp_adapters (shipped)

Priority

P2 — ecosystem extensibility. Today external authors must vendor into the repo.