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/hidden-thinking-label.ts

@@ -1,53 +0,0 @@
-/**
- * Hidden Thinking Label Extension
- *
- * Demonstrates `ctx.ui.setHiddenThinkingLabel()` for customizing the label shown
- * when thinking blocks are hidden.
- *
- * Usage:
- *   pi --extension examples/extensions/hidden-thinking-label.ts
- *
- * Test:
- *   1. Load this extension
- *   2. Hide thinking blocks with Ctrl+T
- *   3. Ask for something that produces reasoning output
- *   4. The collapsed thinking block label will show the custom text
- *
- * Commands:
- *   /thinking-label <text>   Set a custom hidden thinking label
- *   /thinking-label          Reset to the default label
- */
-
-import type { ExtensionAPI, ExtensionContext } from "@mariozechner/pi-coding-agent";
-
-const DEFAULT_LABEL = "Pondering...";
-
-export default function (pi: ExtensionAPI) {
-	let label = DEFAULT_LABEL;
-
-	const applyLabel = (ctx: ExtensionContext) => {
-		ctx.ui.setHiddenThinkingLabel(label);
-	};
-
-	pi.on("session_start", async (_event, ctx) => {
-		applyLabel(ctx);
-	});
-
-	pi.registerCommand("thinking-label", {
-		description: "Set the hidden thinking label. Use without args to reset.",
-		handler: async (args, ctx) => {
-			const nextLabel = args.trim();
-
-			if (!nextLabel) {
-				label = DEFAULT_LABEL;
-				ctx.ui.setHiddenThinkingLabel();
-				ctx.ui.notify(`Hidden thinking label reset to: ${DEFAULT_LABEL}`);
-				return;
-			}
-
-			label = nextLabel;
-			ctx.ui.setHiddenThinkingLabel(label);
-			ctx.ui.notify(`Hidden thinking label set to: ${label}`);
-		},
-	});
-}