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/message-renderer.ts

@@ -1,59 +0,0 @@
-/**
- * Custom message rendering example.
- *
- * Shows how to use registerMessageRenderer to control how custom messages
- * appear in the TUI, with colors, formatting, and expandable details.
- *
- * Usage: /status [message] - sends a status message with custom rendering
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-import { Box, Text } from "@mariozechner/pi-tui";
-
-export default function (pi: ExtensionAPI) {
-	// Register custom renderer for "status-update" messages
-	pi.registerMessageRenderer("status-update", (message, { expanded }, theme) => {
-		const details = message.details as { level: string; timestamp: number } | undefined;
-		const level = details?.level ?? "info";
-
-		// Color based on level
-		const color = level === "error" ? "error" : level === "warn" ? "warning" : "success";
-		const prefix = theme.fg(color, `[${level.toUpperCase()}]`);
-
-		let text = `${prefix} ${message.content}`;
-
-		// Show timestamp when expanded
-		if (expanded && details?.timestamp) {
-			const time = new Date(details.timestamp).toLocaleTimeString();
-			text += `\n${theme.fg("dim", `  at ${time}`)}`;
-		}
-
-		// Use Box with customMessageBg for consistent styling
-		const box = new Box(1, 1, (t) => theme.bg("customMessageBg", t));
-		box.addChild(new Text(text, 0, 0));
-		return box;
-	});
-
-	// Command to send status messages
-	pi.registerCommand("status", {
-		description: "Send a status message (usage: /status [warn|error] message)",
-		handler: async (args, _ctx) => {
-			const parts = args.trim().split(/\s+/);
-			let level = "info";
-			let content = args.trim();
-
-			// Check for level prefix
-			if (parts[0] === "warn" || parts[0] === "error") {
-				level = parts[0];
-				content = parts.slice(1).join(" ") || "Status update";
-			}
-
-			pi.sendMessage({
-				customType: "status-update",
-				content,
-				display: true,
-				details: { level, timestamp: Date.now() },
-			});
-		},
-	});
-}