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/10-settings.ts

@@ -1,51 +0,0 @@
-/**
- * Settings Configuration
- *
- * Override settings using SettingsManager.
- */
-
-import { createAgentSession, SessionManager, SettingsManager } from "@mariozechner/pi-coding-agent";
-
-// Load current settings (merged global + project)
-const settingsManagerFromDisk = SettingsManager.create();
-console.log("Current settings:", JSON.stringify(settingsManagerFromDisk.getGlobalSettings(), null, 2));
-
-// Override specific settings
-const settingsManager = SettingsManager.create();
-settingsManager.applyOverrides({
-	compaction: { enabled: false },
-	retry: { enabled: true, maxRetries: 5, baseDelayMs: 1000 },
-});
-
-await createAgentSession({
-	settingsManager,
-	sessionManager: SessionManager.inMemory(),
-});
-
-console.log("Session created with custom settings");
-
-// Setters update memory immediately and queue persistence writes.
-// Call flush() when you need a durability boundary.
-settingsManager.setDefaultThinkingLevel("low");
-await settingsManager.flush();
-
-// Surface settings I/O errors at the app layer.
-const settingsErrors = settingsManager.drainErrors();
-if (settingsErrors.length > 0) {
-	for (const { scope, error } of settingsErrors) {
-		console.warn(`Warning (${scope} settings): ${error.message}`);
-	}
-}
-
-// For testing without file I/O:
-const inMemorySettings = SettingsManager.inMemory({
-	compaction: { enabled: false },
-	retry: { enabled: false },
-});
-
-await createAgentSession({
-	settingsManager: inMemorySettings,
-	sessionManager: SessionManager.inMemory(),
-});
-
-console.log("Test session created with in-memory settings");