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/agent/test/utils/calculate.ts

@@ -1,32 +0,0 @@
-import { type Static, Type } from "@sinclair/typebox";
-import type { AgentTool, AgentToolResult } from "../../src/types.js";
-
-export interface CalculateResult extends AgentToolResult<undefined> {
-	content: Array<{ type: "text"; text: string }>;
-	details: undefined;
-}
-
-export function calculate(expression: string): CalculateResult {
-	try {
-		const result = new Function(`return ${expression}`)();
-		return { content: [{ type: "text", text: `${expression} = ${result}` }], details: undefined };
-	} catch (e: any) {
-		throw new Error(e.message || String(e));
-	}
-}
-
-const calculateSchema = Type.Object({
-	expression: Type.String({ description: "The mathematical expression to evaluate" }),
-});
-
-type CalculateParams = Static<typeof calculateSchema>;
-
-export const calculateTool: AgentTool<typeof calculateSchema, undefined> = {
-	label: "Calculator",
-	name: "calculate",
-	description: "Evaluate mathematical expressions",
-	parameters: calculateSchema,
-	execute: async (_toolCallId: string, args: CalculateParams) => {
-		return calculate(args.expression);
-	},
-};