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/input-transform.ts

@@ -1,43 +0,0 @@
-/**
- * Input Transform Example - demonstrates the `input` event for intercepting user input.
- *
- * Start pi with this extension:
- *   pi -e ./examples/extensions/input-transform.ts
- *
- * Then type these inside pi:
- *   ?quick What is TypeScript?  → "Respond briefly: What is TypeScript?"
- *   ping                        → "pong" (instant, no LLM)
- *   time                        → current time (instant, no LLM)
- */
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	pi.on("input", async (event, ctx) => {
-		// Source-based logic: skip processing for extension-injected messages
-		if (event.source === "extension") {
-			return { action: "continue" };
-		}
-
-		// Transform: ?quick prefix for brief responses
-		if (event.text.startsWith("?quick ")) {
-			const query = event.text.slice(7).trim();
-			if (!query) {
-				ctx.ui.notify("Usage: ?quick <question>", "warning");
-				return { action: "handled" };
-			}
-			return { action: "transform", text: `Respond briefly in 1-2 sentences: ${query}` };
-		}
-
-		// Handle: instant responses without LLM (extension shows its own feedback)
-		if (event.text.toLowerCase() === "ping") {
-			ctx.ui.notify("pong", "info");
-			return { action: "handled" };
-		}
-		if (event.text.toLowerCase() === "time") {
-			ctx.ui.notify(new Date().toLocaleString(), "info");
-			return { action: "handled" };
-		}
-
-		return { action: "continue" };
-	});
-}