expert-need-detectorlisted

## What It Does Reads the request and the pipeline's S0 grounding slice and decides **which domain experts the spec needs** — a database expert when the request touches the data model, a security expert when it touches auth or a trust boundary, a UI expert when it touches a screen, and so on. It returns a small JSON artifact naming each needed expert and, in one plain sentence, why it fired. This is the model call that **replaces a deterministic signal-scorer** (issue #521, the "one change"): a script cannot reliably tell which expert a request needs — file-path and keyword heuristics emit false signals both ways — so the judgement is the model's. The script's job is only to VALIDATE the result against the roster, never to make it. Nothing needed ⇒ an empty list ⇒ zero experts, zero cost. ## Use This When - The spec pipeline is running with the expert roster enabled (`spec_pipeline_experts_enabled`), after S0 grounding has produced `grounding.json` and before the craft step. Do **not** run this when the experts flag is off — with it off the pipeline is byte-identical to v1 and this skill never runs. ## Inputs - `request_text` — required. The request being specced. - `grounding` — required. The S0 `grounding.json` (references + business terms) — the evidence for which areas the request touches. Decide from THIS, not from the whole repo. - `roster` — optional. The allowed expert roles; defaults to the framework roster. You may name **only** roles in it. The roster