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/session-manager/save-entry.test.ts

@@ -1,55 +0,0 @@
-import { describe, expect, it } from "vitest";
-import { type CustomEntry, SessionManager } from "../../src/core/session-manager.js";
-
-describe("SessionManager.saveCustomEntry", () => {
-	it("saves custom entries and includes them in tree traversal", () => {
-		const session = SessionManager.inMemory();
-
-		// Save a message
-		const msgId = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
-
-		// Save a custom entry
-		const customId = session.appendCustomEntry("my_data", { foo: "bar" });
-
-		// Save another message
-		const msg2Id = session.appendMessage({
-			role: "assistant",
-			content: [{ type: "text", text: "hi" }],
-			api: "anthropic-messages",
-			provider: "anthropic",
-			model: "test",
-			usage: {
-				input: 1,
-				output: 1,
-				cacheRead: 0,
-				cacheWrite: 0,
-				totalTokens: 2,
-				cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
-			},
-			stopReason: "stop",
-			timestamp: 2,
-		});
-
-		// Custom entry should be in entries
-		const entries = session.getEntries();
-		expect(entries).toHaveLength(3);
-
-		const customEntry = entries.find((e) => e.type === "custom") as CustomEntry;
-		expect(customEntry).toBeDefined();
-		expect(customEntry.customType).toBe("my_data");
-		expect(customEntry.data).toEqual({ foo: "bar" });
-		expect(customEntry.id).toBe(customId);
-		expect(customEntry.parentId).toBe(msgId);
-
-		// Tree structure should be correct
-		const path = session.getBranch();
-		expect(path).toHaveLength(3);
-		expect(path[0].id).toBe(msgId);
-		expect(path[1].id).toBe(customId);
-		expect(path[2].id).toBe(msg2Id);
-
-		// buildSessionContext should work (custom entries skipped in messages)
-		const ctx = session.buildSessionContext();
-		expect(ctx.messages).toHaveLength(2); // only message entries
-	});
-});