fix(gate): auto-finalize session after last phase approval

The model sometimes stops after receiving 'Fase approvata' without calling
`tht session finalize`, leaving the session open. Now the gate itself calls
finalize after advancing the max phase (F8), making session closure
deterministic regardless of model behavior.

- reviewer_confirm kind:phase: after phase advance at max_phase, gate calls
  `tht session finalize <session>` (best-effort with recovery message)
- SKILL.md updated: model no longer needs to call finalize itself
- Tests: 2 new JS tests (auto-finalize at max phase; no-finalize at non-max)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -957,6 +957,23 @@ export default function (pi) {
 					PHASE_RECOVERY,
 				);
 				if (err) return err;
+				// Auto-finalize after advancing the LAST phase: the model may not
+				// follow through (token budget, turn end) leaving the session open.
+				const meta = phaseMeta(ctx);
+				if (curNum >= meta.max_phase) {
+					const fErr = relayIfThtFails(
+						ctx,
+						["session", "finalize", session],
+						"Fase approvata ma finalizzazione fallita: esegui manualmente " +
+							`\`tht session finalize ${session}\`.`,
+					);
+					if (fErr) return fErr;
+					lockActive = false;
+					lastSteered = false;
+					return textResult(
+						`Fase ${curNum} approvata e sessione finalizzata (${session}).`,
+					);
+				}
 				return textResult(`Fase approvata (sessione ${session}).`);
 			}
 			if (kind === "cte_plan") {