feat: notify when a newer version is available (#333)

**Summary**  
This change introduces a new startup check that notifies users if a
newer `@openai/codex` version is available. To avoid spamming, it writes
a small state file recording the last check time and will only re‑check
once every 24 hours.

**What’s Changed**  
- **New file** `src/utils/check-updates.ts`  
  - Runs `npm outdated --global @openai/codex`  
  - Reads/writes `codex-state.json` under `CONFIG_DIR`  
  - Limits checks to once per day (`UPDATE_CHECK_FREQUENCY = 24h`)  
- Uses `boxen` for a styled alert and `which` to locate the npm binary
- **Hooked into** `src/cli.tsx` entrypoint:
  ```ts
  import { checkForUpdates } from "./utils/check-updates";
  // …
  // after loading config
  await checkForUpdates().catch();
  ```
- **Dependencies**  
  - Added `boxen@^8.0.1`, `which@^5.0.0`, `@types/which@^3.0.4`  
- **Tests**  
  - Vitest suite under `tests/check-updates.test.ts`  
  - Snapshot in `__snapshots__/check-updates.test.ts.snap`  

**Motivation**  
Addresses issue #244. Users running a stale global install will now see
a friendly reminder—at most once per day—to upgrade and enjoy the latest
features.

**Test Plan**  
- `getNPMCommandPath()` resolves npm correctly  
- `checkOutdated()` parses `npm outdated` JSON  
- State file prevents repeat alerts within 24h  
- Boxen snapshot matches expected output  
- No console output when state indicates a recent check  

**Related Issue**  
try resolves #244


**Preview**
Prompt a pnpm‑style alert when outdated  

![outdated‑alert](https://github.com/user-attachments/assets/294dad45-d858-45d1-bf34-55e672ab883a)

Let me know if you’d tweak any of the messaging, throttle frequency,
placement in the startup flow, or anything else.

---------

Co-authored-by: Thibault Sottiaux <tibo@openai.com>

codex-cli/package.json

@@ -62,8 +62,10 @@
     "@types/marked-terminal": "^6.1.1",
     "@types/react": "^18.0.32",
     "@types/shell-quote": "^1.7.5",
+    "@types/which": "^3.0.4",
     "@typescript-eslint/eslint-plugin": "^7.18.0",
     "@typescript-eslint/parser": "^7.18.0",
+    "boxen": "^8.0.1",
     "esbuild": "^0.25.2",
     "eslint-plugin-import": "^2.31.0",
     "eslint-plugin-react": "^7.32.2",
@@ -77,7 +79,8 @@
     "ts-node": "^10.9.1",
     "typescript": "^5.0.3",
     "vitest": "^3.0.9",
-    "whatwg-url": "^14.2.0"
+    "whatwg-url": "^14.2.0",
+    "which": "^5.0.0"
   },
   "repository": {
     "type": "git",