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/test/initial-message.test.ts

@@ -1,48 +0,0 @@
-import { describe, expect, test } from "vitest";
-import type { Args } from "../src/cli/args.js";
-import { buildInitialMessage } from "../src/cli/initial-message.js";
-
-function createArgs(messages: string[] = []): Args {
-	return {
-		messages: [...messages],
-		fileArgs: [],
-		unknownFlags: new Map(),
-		diagnostics: [],
-	};
-}
-
-describe("buildInitialMessage", () => {
-	test("merges piped stdin with the first CLI message into one prompt", () => {
-		const parsed = createArgs(["Summarize the text given"]);
-		const result = buildInitialMessage({
-			parsed,
-			stdinContent: "README contents\n",
-		});
-
-		expect(result.initialMessage).toBe("README contents\nSummarize the text given");
-		expect(parsed.messages).toEqual([]);
-	});
-
-	test("uses stdin as the initial prompt when no CLI message is present", () => {
-		const parsed = createArgs();
-		const result = buildInitialMessage({
-			parsed,
-			stdinContent: "README contents",
-		});
-
-		expect(result.initialMessage).toBe("README contents");
-		expect(parsed.messages).toEqual([]);
-	});
-
-	test("combines stdin, file text, and first CLI message in one prompt", () => {
-		const parsed = createArgs(["Explain it", "Second message"]);
-		const result = buildInitialMessage({
-			parsed,
-			stdinContent: "stdin\n",
-			fileText: "file\n",
-		});
-
-		expect(result.initialMessage).toBe("stdin\nfile\nExplain it");
-		expect(parsed.messages).toEqual(["Second message"]);
-	});
-});