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/file-trigger.ts

@@ -1,41 +0,0 @@
-/**
- * File Trigger Extension
- *
- * Watches a trigger file and injects its contents into the conversation.
- * Useful for external systems to send messages to the agent.
- *
- * Usage:
- *   echo "Run the tests" > /tmp/agent-trigger.txt
- */
-
-import * as fs from "node:fs";
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	pi.on("session_start", async (_event, ctx) => {
-		const triggerFile = "/tmp/agent-trigger.txt";
-
-		fs.watch(triggerFile, () => {
-			try {
-				const content = fs.readFileSync(triggerFile, "utf-8").trim();
-				if (content) {
-					pi.sendMessage(
-						{
-							customType: "file-trigger",
-							content: `External trigger: ${content}`,
-							display: true,
-						},
-						{ triggerTurn: true }, // triggerTurn - get LLM to respond
-					);
-					fs.writeFileSync(triggerFile, ""); // Clear after reading
-				}
-			} catch {
-				// File might not exist yet
-			}
-		});
-
-		if (ctx.hasUI) {
-			ctx.ui.notify(`Watching ${triggerFile}`, "info");
-		}
-	});
-}