refactor(site): rename Organisation to Instance

The signup form now posts instance_name, matching sitesvc.

Also restores site/next-env.d.ts. Its /// <reference> directives had been
stripped, which removed the Next.js type environment and failed the build
with "Cannot find name 'Promise'". Same cause as the agent's missing build
constraint.

site/app/page.tsx

@@ -14,7 +14,7 @@ export default function OverviewPage() {
                     </p>
                     <div className="hero__acts">
                         <Link className="btn btn--solid" href="/start">
-                            Create your organisation
+                            Create your instance
                         </Link>
                         <Link className="btn btn--line" href="/platform">
                             What it does
@@ -133,8 +133,8 @@ export default function OverviewPage() {
                 <div className="flow">
                     <div className="flow__c">
                         <span className="flow__n">FIRST</span>
-                        <h3>Create an organisation</h3>
-                        <p>You become the owner. Everything inside is invisible to every other organisation.</p>
+                        <h3>Create an instance</h3>
+                        <p>You become the owner. Everything inside is invisible to every other instance.</p>
                     </div>
                     <div className="flow__c">
                         <span className="flow__n">THEN</span>
@@ -168,7 +168,7 @@ export default function OverviewPage() {
                 <div className="card card--cta">
                     <div style={{ maxWidth: "48ch" }}>
                         <h2 style={{ fontSize: "var(--s-1)" }}>Three servers, free, no card.</h2>
-                        <p style={{ color: "var(--ink-2)", marginTop: "0.4rem", fontSize: "0.94rem" }}>Create an organisation, install one agent, and watch a key land on a real box.</p>
+                        <p style={{ color: "var(--ink-2)", marginTop: "0.4rem", fontSize: "0.94rem" }}>Create an instance, install one agent, and watch a key land on a real box.</p>
                     </div>
                     <Link className="btn btn--solid" href="/start">
                         Get started