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/02-custom-model.ts

@@ -1,49 +0,0 @@
-/**
- * Custom Model Selection
- *
- * Shows how to select a specific model and thinking level.
- */
-
-import { getModel } from "@mariozechner/pi-ai";
-import { AuthStorage, createAgentSession, ModelRegistry } from "@mariozechner/pi-coding-agent";
-
-// Set up auth storage and model registry
-const authStorage = AuthStorage.create();
-const modelRegistry = ModelRegistry.create(authStorage);
-
-// Option 1: Find a specific built-in model by provider/id
-const opus = getModel("anthropic", "claude-opus-4-5");
-if (opus) {
-	console.log(`Found model: ${opus.provider}/${opus.id}`);
-}
-
-// Option 2: Find model via registry (includes custom models from models.json)
-const customModel = modelRegistry.find("my-provider", "my-model");
-if (customModel) {
-	console.log(`Found custom model: ${customModel.provider}/${customModel.id}`);
-}
-
-// Option 3: Pick from available models (have valid API keys)
-const available = await modelRegistry.getAvailable();
-console.log(
-	"Available models:",
-	available.map((m) => `${m.provider}/${m.id}`),
-);
-
-if (available.length > 0) {
-	const { session } = await createAgentSession({
-		model: available[0],
-		thinkingLevel: "medium", // off, low, medium, high
-		authStorage,
-		modelRegistry,
-	});
-
-	session.subscribe((event) => {
-		if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
-			process.stdout.write(event.assistantMessageEvent.delta);
-		}
-	});
-
-	await session.prompt("Say hello in one sentence.");
-	console.log();
-}