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/permission-gate.ts

@@ -1,34 +0,0 @@
-/**
- * Permission Gate Extension
- *
- * Prompts for confirmation before running potentially dangerous bash commands.
- * Patterns checked: rm -rf, sudo, chmod/chown 777
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	const dangerousPatterns = [/\brm\s+(-rf?|--recursive)/i, /\bsudo\b/i, /\b(chmod|chown)\b.*777/i];
-
-	pi.on("tool_call", async (event, ctx) => {
-		if (event.toolName !== "bash") return undefined;
-
-		const command = event.input.command as string;
-		const isDangerous = dangerousPatterns.some((p) => p.test(command));
-
-		if (isDangerous) {
-			if (!ctx.hasUI) {
-				// In non-interactive mode, block by default
-				return { block: true, reason: "Dangerous command blocked (no UI for confirmation)" };
-			}
-
-			const choice = await ctx.ui.select(`⚠️ Dangerous command:\n\n  ${command}\n\nAllow?`, ["Yes", "No"]);
-
-			if (choice !== "Yes") {
-				return { block: true, reason: "Blocked by user" };
-			}
-		}
-
-		return undefined;
-	});
-}