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/status-line.ts

@@ -1,32 +0,0 @@
-/**
- * Status Line Extension
- *
- * Demonstrates ctx.ui.setStatus() for displaying persistent status text in the footer.
- * Shows turn progress with themed colors.
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	let turnCount = 0;
-
-	pi.on("session_start", async (_event, ctx) => {
-		const theme = ctx.ui.theme;
-		ctx.ui.setStatus("status-demo", theme.fg("dim", "Ready"));
-	});
-
-	pi.on("turn_start", async (_event, ctx) => {
-		turnCount++;
-		const theme = ctx.ui.theme;
-		const spinner = theme.fg("accent", "●");
-		const text = theme.fg("dim", ` Turn ${turnCount}...`);
-		ctx.ui.setStatus("status-demo", spinner + text);
-	});
-
-	pi.on("turn_end", async (_event, ctx) => {
-		const theme = ctx.ui.theme;
-		const check = theme.fg("success", "✓");
-		const text = theme.fg("dim", ` Turn ${turnCount} complete`);
-		ctx.ui.setStatus("status-demo", check + text);
-	});
-}