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/migration.test.ts

@@ -1,78 +0,0 @@
-import { describe, expect, it } from "vitest";
-import { type FileEntry, migrateSessionEntries } from "../../src/core/session-manager.js";
-
-describe("migrateSessionEntries", () => {
-	it("should add id/parentId to v1 entries", () => {
-		const entries: FileEntry[] = [
-			{ type: "session", id: "sess-1", timestamp: "2025-01-01T00:00:00Z", cwd: "/tmp" },
-			{ type: "message", timestamp: "2025-01-01T00:00:01Z", message: { role: "user", content: "hi", timestamp: 1 } },
-			{
-				type: "message",
-				timestamp: "2025-01-01T00:00:02Z",
-				message: {
-					role: "assistant",
-					content: [{ type: "text", text: "hello" }],
-					api: "test",
-					provider: "test",
-					model: "test",
-					usage: { input: 1, output: 1, cacheRead: 0, cacheWrite: 0 },
-					stopReason: "stop",
-					timestamp: 2,
-				},
-			},
-		] as FileEntry[];
-
-		migrateSessionEntries(entries);
-
-		// Header should have version set (v3 is current after hookMessage->custom migration)
-		expect((entries[0] as any).version).toBe(3);
-
-		// Entries should have id/parentId
-		const msg1 = entries[1] as any;
-		const msg2 = entries[2] as any;
-
-		expect(msg1.id).toBeDefined();
-		expect(msg1.id.length).toBe(8);
-		expect(msg1.parentId).toBeNull();
-
-		expect(msg2.id).toBeDefined();
-		expect(msg2.id.length).toBe(8);
-		expect(msg2.parentId).toBe(msg1.id);
-	});
-
-	it("should be idempotent (skip already migrated)", () => {
-		const entries: FileEntry[] = [
-			{ type: "session", id: "sess-1", version: 2, timestamp: "2025-01-01T00:00:00Z", cwd: "/tmp" },
-			{
-				type: "message",
-				id: "abc12345",
-				parentId: null,
-				timestamp: "2025-01-01T00:00:01Z",
-				message: { role: "user", content: "hi", timestamp: 1 },
-			},
-			{
-				type: "message",
-				id: "def67890",
-				parentId: "abc12345",
-				timestamp: "2025-01-01T00:00:02Z",
-				message: {
-					role: "assistant",
-					content: [{ type: "text", text: "hello" }],
-					api: "test",
-					provider: "test",
-					model: "test",
-					usage: { input: 1, output: 1, cacheRead: 0, cacheWrite: 0 },
-					stopReason: "stop",
-					timestamp: 2,
-				},
-			},
-		] as FileEntry[];
-
-		migrateSessionEntries(entries);
-
-		// IDs should be unchanged
-		expect((entries[1] as any).id).toBe("abc12345");
-		expect((entries[2] as any).id).toBe("def67890");
-		expect((entries[2] as any).parentId).toBe("abc12345");
-	});
-});