Phase 5b: core/router

[cjimti]

Part of #1 and #6 (project bootstrap, v0.1, phase 5 split). Blocked by 5a.

Deliverables

core/router/

• Router interface — Narrow(ctx, userMessage, *mcp.Catalog) ([]mcp.Tool, error).
• PassThrough — returns the full catalog; useful for small MCPs and large models.
• ToolkitClassifier — first-pass call against the same provider.Provider asking "which toolkits are likely needed?" then returns those toolkits' tools. Plexara's datahub_* / trino_* / s3_* / memory_* namespaces map cleanly via mcp.Catalog.Toolkits().
• The classifier prompt and toolkit definitions land under docs/prompts/ so they're reviewable and version-controlled.

Tests

• PassThrough correctness (returns input verbatim).
• ToolkitClassifier against provider.FakeProvider with scripted responses for various user messages.
• Edge cases: empty catalog, classifier returns "all", classifier returns unknown toolkit name.

Out of scope

• core/loop (phase 5c)

Branch

feat/bootstrap-router (off main after 5a merges)

Spec: §6.6 of the bootstrap spec in #1.