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/tui/src/undo-stack.ts

@@ -1,28 +0,0 @@
-/**
- * Generic undo stack with clone-on-push semantics.
- *
- * Stores deep clones of state snapshots. Popped snapshots are returned
- * directly (no re-cloning) since they are already detached.
- */
-export class UndoStack<S> {
-	private stack: S[] = [];
-
-	/** Push a deep clone of the given state onto the stack. */
-	push(state: S): void {
-		this.stack.push(structuredClone(state));
-	}
-
-	/** Pop and return the most recent snapshot, or undefined if empty. */
-	pop(): S | undefined {
-		return this.stack.pop();
-	}
-
-	/** Remove all snapshots. */
-	clear(): void {
-		this.stack.length = 0;
-	}
-
-	get length(): number {
-		return this.stack.length;
-	}
-}