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/03-custom-prompt.ts

@@ -1,55 +0,0 @@
-/**
- * Custom System Prompt
- *
- * Shows how to replace or modify the default system prompt.
- */
-
-import { createAgentSession, DefaultResourceLoader, SessionManager } from "@mariozechner/pi-coding-agent";
-
-// Option 1: Replace prompt entirely
-const loader1 = new DefaultResourceLoader({
-	systemPromptOverride: () => `You are a helpful assistant that speaks like a pirate.
-Always end responses with "Arrr!"`,
-	// Needed to avoid DefaultResourceLoader appending APPEND_SYSTEM.md from ~/.pi/agent or <cwd>/.pi.
-	appendSystemPromptOverride: () => [],
-});
-await loader1.reload();
-
-const { session: session1 } = await createAgentSession({
-	resourceLoader: loader1,
-	sessionManager: SessionManager.inMemory(),
-});
-
-session1.subscribe((event) => {
-	if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
-		process.stdout.write(event.assistantMessageEvent.delta);
-	}
-});
-
-console.log("=== Replace prompt ===");
-await session1.prompt("What is 2 + 2?");
-console.log("\n");
-
-// Option 2: Append instructions to the default prompt
-const loader2 = new DefaultResourceLoader({
-	appendSystemPromptOverride: (base) => [
-		...base,
-		"## Additional Instructions\n- Always be concise\n- Use bullet points when listing things",
-	],
-});
-await loader2.reload();
-
-const { session: session2 } = await createAgentSession({
-	resourceLoader: loader2,
-	sessionManager: SessionManager.inMemory(),
-});
-
-session2.subscribe((event) => {
-	if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
-		process.stdout.write(event.assistantMessageEvent.delta);
-	}
-});
-
-console.log("=== Modify prompt ===");
-await session2.prompt("List 3 benefits of TypeScript.");
-console.log();