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/with-deps/index.ts

@@ -1,32 +0,0 @@
-/**
- * Example extension with its own npm dependencies.
- * Tests that jiti resolves modules from the extension's own node_modules.
- *
- * Requires: npm install in this directory
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-import { Type } from "@sinclair/typebox";
-import ms from "ms";
-
-export default function (pi: ExtensionAPI) {
-	// Register a tool that uses ms
-	pi.registerTool({
-		name: "parse_duration",
-		label: "Parse Duration",
-		description: "Parse a human-readable duration string (e.g., '2 days', '1h', '5m') to milliseconds",
-		parameters: Type.Object({
-			duration: Type.String({ description: "Duration string like '2 days', '1h', '5m'" }),
-		}),
-		execute: async (_toolCallId, params) => {
-			const result = ms(params.duration as ms.StringValue);
-			if (result === undefined) {
-				throw new Error(`Invalid duration: "${params.duration}"`);
-			}
-			return {
-				content: [{ type: "text", text: `${params.duration} = ${result} milliseconds` }],
-				details: {},
-			};
-		},
-	});
-}