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/src/core/output-guard.ts

@@ -1,74 +0,0 @@
-interface StdoutTakeoverState {
-	rawStdoutWrite: (chunk: string, callback?: (error?: Error | null) => void) => boolean;
-	rawStderrWrite: (chunk: string, callback?: (error?: Error | null) => void) => boolean;
-	originalStdoutWrite: typeof process.stdout.write;
-}
-
-let stdoutTakeoverState: StdoutTakeoverState | undefined;
-
-export function takeOverStdout(): void {
-	if (stdoutTakeoverState) {
-		return;
-	}
-
-	const rawStdoutWrite = process.stdout.write.bind(process.stdout) as StdoutTakeoverState["rawStdoutWrite"];
-	const rawStderrWrite = process.stderr.write.bind(process.stderr) as StdoutTakeoverState["rawStderrWrite"];
-	const originalStdoutWrite = process.stdout.write;
-
-	process.stdout.write = ((
-		chunk: string | Uint8Array,
-		encodingOrCallback?: BufferEncoding | ((error?: Error | null) => void),
-		callback?: (error?: Error | null) => void,
-	): boolean => {
-		if (typeof encodingOrCallback === "function") {
-			return rawStderrWrite(String(chunk), encodingOrCallback);
-		}
-		return rawStderrWrite(String(chunk), callback);
-	}) as typeof process.stdout.write;
-
-	stdoutTakeoverState = {
-		rawStdoutWrite,
-		rawStderrWrite,
-		originalStdoutWrite,
-	};
-}
-
-export function restoreStdout(): void {
-	if (!stdoutTakeoverState) {
-		return;
-	}
-
-	process.stdout.write = stdoutTakeoverState.originalStdoutWrite;
-	stdoutTakeoverState = undefined;
-}
-
-export function isStdoutTakenOver(): boolean {
-	return stdoutTakeoverState !== undefined;
-}
-
-export function writeRawStdout(text: string): void {
-	if (stdoutTakeoverState) {
-		stdoutTakeoverState.rawStdoutWrite(text);
-		return;
-	}
-	process.stdout.write(text);
-}
-
-export async function flushRawStdout(): Promise<void> {
-	if (stdoutTakeoverState) {
-		await new Promise<void>((resolve, reject) => {
-			stdoutTakeoverState?.rawStdoutWrite("", (err) => {
-				if (err) reject(err);
-				else resolve();
-			});
-		});
-		return;
-	}
-
-	await new Promise<void>((resolve, reject) => {
-		process.stdout.write("", (err) => {
-			if (err) reject(err);
-			else resolve();
-		});
-	});
-}