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/test/compaction-extensions-example.test.ts

@@ -1,66 +0,0 @@
-/**
- * Verify the documentation example from extensions.md compiles and works.
- */
-
-import { describe, expect, it } from "vitest";
-import type { ExtensionAPI, SessionBeforeCompactEvent, SessionCompactEvent } from "../src/core/extensions/index.js";
-
-describe("Documentation example", () => {
-	it("custom compaction example should type-check correctly", () => {
-		// This is the example from extensions.md - verify it compiles
-		const exampleExtension = (pi: ExtensionAPI) => {
-			pi.on("session_before_compact", async (event: SessionBeforeCompactEvent, ctx) => {
-				// All these should be accessible on the event
-				const { preparation, branchEntries } = event;
-				// sessionManager, modelRegistry, and model come from ctx
-				const { sessionManager, modelRegistry } = ctx;
-				const { messagesToSummarize, turnPrefixMessages, tokensBefore, firstKeptEntryId, isSplitTurn } =
-					preparation;
-
-				// Verify types
-				expect(Array.isArray(messagesToSummarize)).toBe(true);
-				expect(Array.isArray(turnPrefixMessages)).toBe(true);
-				expect(typeof isSplitTurn).toBe("boolean");
-				expect(typeof tokensBefore).toBe("number");
-				expect(typeof sessionManager.getEntries).toBe("function");
-				expect(typeof modelRegistry.getApiKeyAndHeaders).toBe("function");
-				expect(typeof firstKeptEntryId).toBe("string");
-				expect(Array.isArray(branchEntries)).toBe(true);
-
-				const summary = messagesToSummarize
-					.filter((m) => m.role === "user")
-					.map((m) => `- ${typeof m.content === "string" ? m.content.slice(0, 100) : "[complex]"}`)
-					.join("\n");
-
-				// Extensions return compaction content - SessionManager adds id/parentId
-				return {
-					compaction: {
-						summary: `User requests:\n${summary}`,
-						firstKeptEntryId,
-						tokensBefore,
-					},
-				};
-			});
-		};
-
-		// Just verify the function exists and is callable
-		expect(typeof exampleExtension).toBe("function");
-	});
-
-	it("compact event should have correct fields", () => {
-		const checkCompactEvent = (pi: ExtensionAPI) => {
-			pi.on("session_compact", async (event: SessionCompactEvent) => {
-				// These should all be accessible
-				const entry = event.compactionEntry;
-				const fromExtension = event.fromExtension;
-
-				expect(entry.type).toBe("compaction");
-				expect(typeof entry.summary).toBe("string");
-				expect(typeof entry.tokensBefore).toBe("number");
-				expect(typeof fromExtension).toBe("boolean");
-			});
-		};
-
-		expect(typeof checkCompactEvent).toBe("function");
-	});
-});