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/sdk/01-minimal.ts

@@ -1,22 +0,0 @@
-/**
- * Minimal SDK Usage
- *
- * Uses all defaults: discovers skills, extensions, tools, context files
- * from cwd and ~/.pi/agent. Model chosen from settings or first available.
- */
-
-import { createAgentSession } from "@mariozechner/pi-coding-agent";
-
-const { session } = await createAgentSession();
-
-session.subscribe((event) => {
-	if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
-		process.stdout.write(event.assistantMessageEvent.delta);
-	}
-});
-
-await session.prompt("What files are in the current directory?");
-session.state.messages.forEach((msg) => {
-	console.log(msg);
-});
-console.log();