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/auto-commit-on-exit.ts

@@ -1,49 +0,0 @@
-/**
- * Auto-Commit on Exit Extension
- *
- * Automatically commits changes when the agent exits.
- * Uses the last assistant message to generate a commit message.
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	pi.on("session_shutdown", async (_event, ctx) => {
-		// Check for uncommitted changes
-		const { stdout: status, code } = await pi.exec("git", ["status", "--porcelain"]);
-
-		if (code !== 0 || status.trim().length === 0) {
-			// Not a git repo or no changes
-			return;
-		}
-
-		// Find the last assistant message for commit context
-		const entries = ctx.sessionManager.getEntries();
-		let lastAssistantText = "";
-		for (let i = entries.length - 1; i >= 0; i--) {
-			const entry = entries[i];
-			if (entry.type === "message" && entry.message.role === "assistant") {
-				const content = entry.message.content;
-				if (Array.isArray(content)) {
-					lastAssistantText = content
-						.filter((c): c is { type: "text"; text: string } => c.type === "text")
-						.map((c) => c.text)
-						.join("\n");
-				}
-				break;
-			}
-		}
-
-		// Generate a simple commit message
-		const firstLine = lastAssistantText.split("\n")[0] || "Work in progress";
-		const commitMessage = `[pi] ${firstLine.slice(0, 50)}${firstLine.length > 50 ? "..." : ""}`;
-
-		// Stage and commit
-		await pi.exec("git", ["add", "-A"]);
-		const { code: commitCode } = await pi.exec("git", ["commit", "-m", commitMessage]);
-
-		if (commitCode === 0 && ctx.hasUI) {
-			ctx.ui.notify(`Auto-committed: ${commitMessage}`, "info");
-		}
-	});
-}