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/protected-paths.ts

@@ -1,30 +0,0 @@
-/**
- * Protected Paths Extension
- *
- * Blocks write and edit operations to protected paths.
- * Useful for preventing accidental modifications to sensitive files.
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	const protectedPaths = [".env", ".git/", "node_modules/"];
-
-	pi.on("tool_call", async (event, ctx) => {
-		if (event.toolName !== "write" && event.toolName !== "edit") {
-			return undefined;
-		}
-
-		const path = event.input.path as string;
-		const isProtected = protectedPaths.some((p) => path.includes(p));
-
-		if (isProtected) {
-			if (ctx.hasUI) {
-				ctx.ui.notify(`Blocked write to protected path: ${path}`, "warning");
-			}
-			return { block: true, reason: `Path "${path}" is protected` };
-		}
-
-		return undefined;
-	});
-}