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/07-context-files.ts

@@ -1,40 +0,0 @@
-/**
- * Context Files (AGENTS.md)
- *
- * Context files provide project-specific instructions loaded into the system prompt.
- */
-
-import { createAgentSession, DefaultResourceLoader, SessionManager } from "@mariozechner/pi-coding-agent";
-
-// Disable context files entirely by returning an empty list in agentsFilesOverride.
-const loader = new DefaultResourceLoader({
-	agentsFilesOverride: (current) => ({
-		agentsFiles: [
-			...current.agentsFiles,
-			{
-				path: "/virtual/AGENTS.md",
-				content: `# Project Guidelines
-
-## Code Style
-- Use TypeScript strict mode
-- No any types
-- Prefer const over let`,
-			},
-		],
-	}),
-});
-await loader.reload();
-
-// Discover AGENTS.md files walking up from cwd
-const discovered = loader.getAgentsFiles().agentsFiles;
-console.log("Discovered context files:");
-for (const file of discovered) {
-	console.log(`  - ${file.path} (${file.content.length} chars)`);
-}
-
-await createAgentSession({
-	resourceLoader: loader,
-	sessionManager: SessionManager.inMemory(),
-});
-
-console.log(`Session created with ${discovered.length + 1} context files`);