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/trigger-compact.ts

@@ -1,50 +0,0 @@
-import type { ExtensionAPI, ExtensionContext } from "@mariozechner/pi-coding-agent";
-
-const COMPACT_THRESHOLD_TOKENS = 100_000;
-
-export default function (pi: ExtensionAPI) {
-	let previousTokens: number | null | undefined;
-
-	const triggerCompaction = (ctx: ExtensionContext, customInstructions?: string) => {
-		if (ctx.hasUI) {
-			ctx.ui.notify("Compaction started", "info");
-		}
-		ctx.compact({
-			customInstructions,
-			onComplete: () => {
-				if (ctx.hasUI) {
-					ctx.ui.notify("Compaction completed", "info");
-				}
-			},
-			onError: (error) => {
-				if (ctx.hasUI) {
-					ctx.ui.notify(`Compaction failed: ${error.message}`, "error");
-				}
-			},
-		});
-	};
-
-	pi.on("turn_end", (_event, ctx) => {
-		const usage = ctx.getContextUsage();
-		const currentTokens = usage?.tokens ?? null;
-		if (currentTokens === null) {
-			return;
-		}
-
-		const crossedThreshold =
-			previousTokens !== undefined && previousTokens !== null && previousTokens <= COMPACT_THRESHOLD_TOKENS;
-		previousTokens = currentTokens;
-		if (!crossedThreshold || currentTokens <= COMPACT_THRESHOLD_TOKENS) {
-			return;
-		}
-		triggerCompaction(ctx);
-	});
-
-	pi.registerCommand("trigger-compact", {
-		description: "Trigger compaction immediately",
-		handler: async (args, ctx) => {
-			const instructions = args.trim() || undefined;
-			triggerCompaction(ctx, instructions);
-		},
-	});
-}