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/extensions/reload-runtime.ts

@@ -1,37 +0,0 @@
-/**
- * Reload Runtime Extension
- *
- * Demonstrates ctx.reload() from ExtensionCommandContext and an LLM-callable
- * tool that queues a follow-up command to trigger reload.
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-import { Type } from "@sinclair/typebox";
-
-export default function (pi: ExtensionAPI) {
-	// Command entrypoint for reload.
-	// Treat reload as terminal for this handler.
-	pi.registerCommand("reload-runtime", {
-		description: "Reload extensions, skills, prompts, and themes",
-		handler: async (_args, ctx) => {
-			await ctx.reload();
-			return;
-		},
-	});
-
-	// LLM-callable tool. Tools get ExtensionContext, so they cannot call ctx.reload() directly.
-	// Instead, queue a follow-up user command that executes the command above.
-	pi.registerTool({
-		name: "reload_runtime",
-		label: "Reload Runtime",
-		description: "Reload extensions, skills, prompts, and themes",
-		parameters: Type.Object({}),
-		async execute() {
-			pi.sendUserMessage("/reload-runtime", { deliverAs: "followUp" });
-			return {
-				content: [{ type: "text", text: "Queued /reload-runtime as a follow-up command." }],
-				details: {},
-			};
-		},
-	});
-}