feat: show version update notification on CLI startup

[notgitika]

Description

Adds a non-blocking startup notification that checks npm for newer CLI versions and displays a colored notice. Includes 24-hour caching to avoid slowing down every invocation.

This is how it looks like once you exit the TUI

---0

Closes #264

Changes

• src/cli/update-notifier.ts — New module with checkForUpdate() (cache read/write, fetch, compare) and printUpdateNotification() (colored stderr output)
• src/cli/cli.ts — Fire-and-forget update check at startup; print notification after command completes (CLI) or after TUI exits
• src/cli/__tests__/update-notifier.test.ts — 10 unit tests covering cache behavior, error handling, and notification formatting

How it works

1. On every invocation, checkForUpdate() runs concurrently with the actual command
2. Reads ~/.agentcore/update-check.json — skips network call if last check was < 24h ago
3. Fetches latest version from npm via existing fetchLatestVersion()
4. Compares using existing compareVersions() (with prerelease support from fix: handle pre-release versions in compareVersions #357)
5. After the command finishes, prints a colored notice to stderr if an update is available
6. Skipped for agentcore update (redundant)
7. Never throws — all errors silently return null

Test plan

• npm run format:check passes
• npm run lint passes
• npm run typecheck passes
• npm test — 1747 tests pass including 10 new ones
• Manual: run agentcore status with an older version → notification appears
• Manual: run again immediately → uses cache (no network request)
• Manual: agentcore update → no notification
• Manual: agentcore (TUI) → notification appears after exit