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/confirm-destructive.ts

@@ -1,59 +0,0 @@
-/**
- * Confirm Destructive Actions Extension
- *
- * Prompts for confirmation before destructive session actions (clear, switch, branch).
- * Demonstrates how to cancel session events using the before_* events.
- */
-
-import type { ExtensionAPI, SessionBeforeSwitchEvent, SessionMessageEntry } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	pi.on("session_before_switch", async (event: SessionBeforeSwitchEvent, ctx) => {
-		if (!ctx.hasUI) return;
-
-		if (event.reason === "new") {
-			const confirmed = await ctx.ui.confirm(
-				"Clear session?",
-				"This will delete all messages in the current session.",
-			);
-
-			if (!confirmed) {
-				ctx.ui.notify("Clear cancelled", "info");
-				return { cancel: true };
-			}
-			return;
-		}
-
-		// reason === "resume" - check if there are unsaved changes (messages since last assistant response)
-		const entries = ctx.sessionManager.getEntries();
-		const hasUnsavedWork = entries.some(
-			(e): e is SessionMessageEntry => e.type === "message" && e.message.role === "user",
-		);
-
-		if (hasUnsavedWork) {
-			const confirmed = await ctx.ui.confirm(
-				"Switch session?",
-				"You have messages in the current session. Switch anyway?",
-			);
-
-			if (!confirmed) {
-				ctx.ui.notify("Switch cancelled", "info");
-				return { cancel: true };
-			}
-		}
-	});
-
-	pi.on("session_before_fork", async (event, ctx) => {
-		if (!ctx.hasUI) return;
-
-		const choice = await ctx.ui.select(`Fork from entry ${event.entryId.slice(0, 8)}?`, [
-			"Yes, create fork",
-			"No, stay in current session",
-		]);
-
-		if (choice !== "Yes, create fork") {
-			ctx.ui.notify("Fork cancelled", "info");
-			return { cancel: true };
-		}
-	});
-}