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/09-api-keys-and-oauth.ts

@@ -1,48 +0,0 @@
-/**
- * API Keys and OAuth
- *
- * Configure API key resolution via AuthStorage and ModelRegistry.
- */
-
-import { AuthStorage, createAgentSession, ModelRegistry, SessionManager } from "@mariozechner/pi-coding-agent";
-
-// Default: AuthStorage uses ~/.pi/agent/auth.json
-// ModelRegistry loads built-in + custom models from ~/.pi/agent/models.json
-const authStorage = AuthStorage.create();
-const modelRegistry = ModelRegistry.create(authStorage);
-
-await createAgentSession({
-	sessionManager: SessionManager.inMemory(),
-	authStorage,
-	modelRegistry,
-});
-console.log("Session with default auth storage and model registry");
-
-// Custom auth storage location
-const customAuthStorage = AuthStorage.create("/tmp/my-app/auth.json");
-const customModelRegistry = ModelRegistry.create(customAuthStorage, "/tmp/my-app/models.json");
-
-await createAgentSession({
-	sessionManager: SessionManager.inMemory(),
-	authStorage: customAuthStorage,
-	modelRegistry: customModelRegistry,
-});
-console.log("Session with custom auth storage location");
-
-// Runtime API key override (not persisted to disk)
-authStorage.setRuntimeApiKey("anthropic", "sk-my-temp-key");
-await createAgentSession({
-	sessionManager: SessionManager.inMemory(),
-	authStorage,
-	modelRegistry,
-});
-console.log("Session with runtime API key override");
-
-// No models.json - only built-in models
-const simpleRegistry = ModelRegistry.inMemory(authStorage);
-await createAgentSession({
-	sessionManager: SessionManager.inMemory(),
-	authStorage,
-	modelRegistry: simpleRegistry,
-});
-console.log("Session with only built-in models");