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/08-prompt-templates.ts

@@ -1,48 +0,0 @@
-/**
- * Prompt Templates
- *
- * File-based templates that inject content when invoked with /templatename.
- */
-
-import {
-	createAgentSession,
-	createSyntheticSourceInfo,
-	DefaultResourceLoader,
-	type PromptTemplate,
-	SessionManager,
-} from "@mariozechner/pi-coding-agent";
-
-// Define custom templates
-const deployTemplate: PromptTemplate = {
-	name: "deploy",
-	description: "Deploy the application",
-	filePath: "/virtual/prompts/deploy.md",
-	sourceInfo: createSyntheticSourceInfo("/virtual/prompts/deploy.md", { source: "sdk" }),
-	content: `# Deploy Instructions
-
-1. Build: npm run build
-2. Test: npm test
-3. Deploy: npm run deploy`,
-};
-
-const loader = new DefaultResourceLoader({
-	promptsOverride: (current) => ({
-		prompts: [...current.prompts, deployTemplate],
-		diagnostics: current.diagnostics,
-	}),
-});
-await loader.reload();
-
-// Discover templates from cwd/.pi/prompts/ and ~/.pi/agent/prompts/
-const discovered = loader.getPrompts().prompts;
-console.log("Discovered prompt templates:");
-for (const template of discovered) {
-	console.log(`  /${template.name}: ${template.description}`);
-}
-
-await createAgentSession({
-	resourceLoader: loader,
-	sessionManager: SessionManager.inMemory(),
-});
-
-console.log(`Session created with ${discovered.length + 1} prompt templates`);