chore: drop non-code junk (pi-mono-reference, runtime state, root duplicates)

Remove files that should never have been tracked:

- pi-mono-reference/: 688-file copy of an upstream monorepo, zero imports,
  used only as offline human reference
- build/electron-builder.runtime.json: environment-specific build artifact
  with hardcoded local paths
- state/cancel-signal-state.json: runtime state written by the app
- prompt.txt, logo.svg (root): exact duplicates of the canonical copies
  under .planning/archive/ and assets/

Add state/ to .gitignore so future runtime state stays out.

pi-mono-reference/packages/coding-agent/examples/sdk/13-session-runtime.ts

@@ -1,67 +0,0 @@
-/**
- * Session runtime
- *
- * Use AgentSessionRuntime when you need to replace the active AgentSession,
- * for example for new-session, resume, fork, or import flows.
- *
- * The important pattern is: after the runtime replaces the active session,
- * rebind any session-local subscriptions and extension bindings to `runtime.session`.
- */
-
-import {
-	type CreateAgentSessionRuntimeFactory,
-	createAgentSessionFromServices,
-	createAgentSessionRuntime,
-	createAgentSessionServices,
-	getAgentDir,
-	SessionManager,
-} from "@mariozechner/pi-coding-agent";
-
-const createRuntime: CreateAgentSessionRuntimeFactory = async ({ cwd, sessionManager, sessionStartEvent }) => {
-	const services = await createAgentSessionServices({ cwd });
-	return {
-		...(await createAgentSessionFromServices({
-			services,
-			sessionManager,
-			sessionStartEvent,
-		})),
-		services,
-		diagnostics: services.diagnostics,
-	};
-};
-const runtime = await createAgentSessionRuntime(createRuntime, {
-	cwd: process.cwd(),
-	agentDir: getAgentDir(),
-	sessionManager: SessionManager.create(process.cwd()),
-});
-
-let unsubscribe: (() => void) | undefined;
-
-async function bindSession() {
-	unsubscribe?.();
-	const session = runtime.session;
-	await session.bindExtensions({});
-	unsubscribe = session.subscribe((event) => {
-		if (event.type === "queue_update") {
-			console.log("Queued:", event.steering.length + event.followUp.length);
-		}
-	});
-	return session;
-}
-
-let session = await bindSession();
-const originalSessionFile = session.sessionFile;
-console.log("Initial session:", originalSessionFile);
-
-await runtime.newSession();
-session = await bindSession();
-console.log("After newSession():", session.sessionFile);
-
-if (originalSessionFile) {
-	await runtime.switchSession(originalSessionFile);
-	session = await bindSession();
-	console.log("After switchSession():", session.sessionFile);
-}
-
-unsubscribe?.();
-await runtime.dispose();