feat(harness): SQL formula evidence -- concept->formula units + retrieval (D14b)

New evidence/formula_store.py: ConceptFormula (concept, columns, sql, status,
sources) as a frontmatter-YAML + SQL-body unit, stored one-file-per-formula under
<formulas>/<slug>-<n>.sql.md. save_formula is append-only (competing drafts and
reviewed versions coexist); retrieve_formula(concept) returns all of them so the
gate can surface candidates and let the reviewer choose.

concept_formula_approved / concept_formula_rejected added to DecisionType
(records the reviewer's choice; approved formulas travel with schema-linking).

L1: test_formula (7 tests) -- retrieval by concept, save/reload roundtrip (SQL
body preserved, frontmatter well-formed), multiple formulas per concept, empty
on no-match / missing dir, decision-type existence, default draft status.

Deferred: --kind formula on nsp search (needs search_cmd porting) wires
retrieve_formula into the CLI; lands with the search command.

harness/nsp/decisions.py

@@ -39,6 +39,11 @@ DecisionType = Literal[
     # "phase:4", detail = il valore (es. "ablazione"), rationale = la/e colonna/e scelta/e
     # dal reviewer (aggregate_lsh_multi le espone tutte senza collassare al miglior match).
     "value_grounded",
+    # D14b: formula di concetto approvata/rifiutata dal reviewer. subject = "phase:4",
+    # detail = il concetto (es. "fascia pediatrica"), rationale = la/e colonna/e o il motivo.
+    # retrieve_formula restituisce i candidati; queste decisioni registrano la scelta.
+    "concept_formula_approved",
+    "concept_formula_rejected",
 ]