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/git-checkpoint.ts

@@ -1,53 +0,0 @@
-/**
- * Git Checkpoint Extension
- *
- * Creates git stash checkpoints at each turn so /fork can restore code state.
- * When forking, offers to restore code to that point in history.
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	const checkpoints = new Map<string, string>();
-	let currentEntryId: string | undefined;
-
-	// Track the current entry ID when user messages are saved
-	pi.on("tool_result", async (_event, ctx) => {
-		const leaf = ctx.sessionManager.getLeafEntry();
-		if (leaf) currentEntryId = leaf.id;
-	});
-
-	pi.on("turn_start", async () => {
-		// Create a git stash entry before LLM makes changes
-		const { stdout } = await pi.exec("git", ["stash", "create"]);
-		const ref = stdout.trim();
-		if (ref && currentEntryId) {
-			checkpoints.set(currentEntryId, ref);
-		}
-	});
-
-	pi.on("session_before_fork", async (event, ctx) => {
-		const ref = checkpoints.get(event.entryId);
-		if (!ref) return;
-
-		if (!ctx.hasUI) {
-			// In non-interactive mode, don't restore automatically
-			return;
-		}
-
-		const choice = await ctx.ui.select("Restore code state?", [
-			"Yes, restore code to that point",
-			"No, keep current code",
-		]);
-
-		if (choice?.startsWith("Yes")) {
-			await pi.exec("git", ["stash", "apply", ref]);
-			ctx.ui.notify("Code restored to checkpoint", "info");
-		}
-	});
-
-	pi.on("agent_end", async () => {
-		// Clear checkpoints after agent completes
-		checkpoints.clear();
-	});
-}