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/model-status.ts

@@ -1,31 +0,0 @@
-/**
- * Model status extension - shows model changes in the status bar.
- *
- * Demonstrates the `model_select` hook which fires when the model changes
- * via /model command, Ctrl+P cycling, or session restore.
- *
- * Usage: pi -e ./model-status.ts
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	pi.on("model_select", async (event, ctx) => {
-		const { model, previousModel, source } = event;
-
-		// Format model identifiers
-		const next = `${model.provider}/${model.id}`;
-		const prev = previousModel ? `${previousModel.provider}/${previousModel.id}` : "none";
-
-		// Show notification on change
-		if (source !== "restore") {
-			ctx.ui.notify(`Model: ${next}`, "info");
-		}
-
-		// Update status bar with current model
-		ctx.ui.setStatus("model", `🤖 ${model.id}`);
-
-		// Log change details (visible in debug output)
-		console.log(`[model_select] ${prev} → ${next} (${source})`);
-	});
-}