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/extensions/dynamic-resources/index.ts

@@ -1,15 +0,0 @@
-import { dirname, join } from "node:path";
-import { fileURLToPath } from "node:url";
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-const baseDir = dirname(fileURLToPath(import.meta.url));
-
-export default function (pi: ExtensionAPI) {
-	pi.on("resources_discover", () => {
-		return {
-			skillPaths: [join(baseDir, "SKILL.md")],
-			promptPaths: [join(baseDir, "dynamic.md")],
-			themePaths: [join(baseDir, "dynamic.json")],
-		};
-	});
-}