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/session-name.ts

@@ -1,27 +0,0 @@
-/**
- * Session naming example.
- *
- * Shows setSessionName/getSessionName to give sessions friendly names
- * that appear in the session selector instead of the first message.
- *
- * Usage: /session-name [name] - set or show session name
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	pi.registerCommand("session-name", {
-		description: "Set or show session name (usage: /session-name [new name])",
-		handler: async (args, ctx) => {
-			const name = args.trim();
-
-			if (name) {
-				pi.setSessionName(name);
-				ctx.ui.notify(`Session named: ${name}`, "info");
-			} else {
-				const current = pi.getSessionName();
-				ctx.ui.notify(current ? `Session: ${current}` : "No session name set", "info");
-			}
-		},
-	});
-}