feat(gate): write_schema_linking tool (validated F4 artifact via CLI)

tht() gains an optional stdin arg; the tool pipes the schema-linking
object to 'tht session set-schema-linking --file -', which validates
against SchemaLinking and returns the exact error on failure — so the
model stops hand-writing the artifact and validating with ad-hoc python.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

harness/.pi/extensions/tht-gate.js

@@ -144,9 +144,9 @@ function loadEnvFromDotenv(ctx) {
 }
 
 // Single chokepoint for all CLI calls. cwd is the Pi project root (harness/).
-function tht(ctx, args) {
+function tht(ctx, args, input) {
 	loadEnvFromDotenv(ctx);
-	return execFileSync("tht", args, { cwd: ctx.cwd, encoding: "utf8" });
+	return execFileSync("tht", args, { cwd: ctx.cwd, encoding: "utf8", input });
 }
 
 // Runs a privileged tht call; converts any failure into an actionable textResult
@@ -701,6 +701,38 @@ export default function (pi) {
 		},
 	});
 
+	pi.registerTool({
+		name: "write_schema_linking",
+		label: "Scrittura schema_linking.json (validata)",
+		description:
+			"Scrive deterministicamente schema_linking.json validandolo contro il modello " +
+			"SchemaLinking via tht session set-schema-linking (evita edit a mano e la " +
+			"validazione manuale). schema_linking e' l'oggetto JSON completo: {question, " +
+			"candidates:[{kind:'table'|'column', name, evidence?, decision?}], joins:[{from, " +
+			"to, source?}], excluded:[{kind, name}], open_questions:[], concept_formulas:[]}.",
+		parameters: Type.Object({
+			session: Type.String(),
+			schema_linking: Type.Any(),
+		}),
+		async execute(_id, params, _signal, _onUpdate, ctx) {
+			lockActive = true;
+			const { session, schema_linking } = params;
+			try {
+				tht(
+					ctx,
+					["session", "set-schema-linking", session, "--file", "-"],
+					JSON.stringify(schema_linking),
+				);
+				return textResult(
+					`schema_linking.json scritto e validato per la sessione ${session}.`,
+				);
+			} catch (e) {
+				const cliMsg = (e.stderr || e.message || String(e)).toString().trim();
+				return textResult(`${cliMsg} Correggi schema_linking e riprova.`);
+			}
+		},
+	});
+
 	// --- slash command: /torna [session_id] [N] (rollback to a previous phase) --
 	pi.registerCommand("torna", {
 		description: