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/pirate.ts

@@ -1,47 +0,0 @@
-/**
- * Pirate Extension
- *
- * Demonstrates modifying the system prompt in before_agent_start to dynamically
- * change agent behavior based on extension state.
- *
- * Usage:
- * 1. Copy this file to ~/.pi/agent/extensions/ or your project's .pi/extensions/
- * 2. Use /pirate to toggle pirate mode
- * 3. When enabled, the agent will respond like a pirate
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function pirateExtension(pi: ExtensionAPI) {
-	let pirateMode = false;
-
-	// Register /pirate command to toggle pirate mode
-	pi.registerCommand("pirate", {
-		description: "Toggle pirate mode (agent speaks like a pirate)",
-		handler: async (_args, ctx) => {
-			pirateMode = !pirateMode;
-			ctx.ui.notify(pirateMode ? "Arrr! Pirate mode enabled!" : "Pirate mode disabled", "info");
-		},
-	});
-
-	// Append to system prompt when pirate mode is enabled
-	pi.on("before_agent_start", async (event) => {
-		if (pirateMode) {
-			return {
-				systemPrompt:
-					event.systemPrompt +
-					`
-
-IMPORTANT: You are now in PIRATE MODE. You must:
-- Speak like a stereotypical pirate in all responses
-- Use phrases like "Arrr!", "Ahoy!", "Shiver me timbers!", "Avast!", "Ye scurvy dog!"
-- Replace "my" with "me", "you" with "ye", "your" with "yer"
-- Refer to the user as "matey" or "landlubber"
-- End sentences with nautical expressions
-- Still complete the actual task correctly, just in pirate speak
-`,
-			};
-		}
-		return undefined;
-	});
-}