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/bookmark.ts

@@ -1,50 +0,0 @@
-/**
- * Entry bookmarking example.
- *
- * Shows setLabel to mark entries with labels for easy navigation in /tree.
- * Labels appear in the tree view and help you find important points.
- *
- * Usage: /bookmark [label] - bookmark the last assistant message
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	pi.registerCommand("bookmark", {
-		description: "Bookmark last message (usage: /bookmark [label])",
-		handler: async (args, ctx) => {
-			const label = args.trim() || `bookmark-${Date.now()}`;
-
-			// Find the last assistant message entry
-			const entries = ctx.sessionManager.getEntries();
-			for (let i = entries.length - 1; i >= 0; i--) {
-				const entry = entries[i];
-				if (entry.type === "message" && entry.message.role === "assistant") {
-					pi.setLabel(entry.id, label);
-					ctx.ui.notify(`Bookmarked as: ${label}`, "info");
-					return;
-				}
-			}
-
-			ctx.ui.notify("No assistant message to bookmark", "warning");
-		},
-	});
-
-	// Remove bookmark
-	pi.registerCommand("unbookmark", {
-		description: "Remove bookmark from last labeled entry",
-		handler: async (_args, ctx) => {
-			const entries = ctx.sessionManager.getEntries();
-			for (let i = entries.length - 1; i >= 0; i--) {
-				const entry = entries[i];
-				const label = ctx.sessionManager.getLabel(entry.id);
-				if (label) {
-					pi.setLabel(entry.id, undefined);
-					ctx.ui.notify(`Removed bookmark: ${label}`, "info");
-					return;
-				}
-			}
-			ctx.ui.notify("No bookmarked entry found", "warning");
-		},
-	});
-}