split authkey setup script and invoke during poststart

[rajsinghtech]

The devcontainer was failing to authenticate automatically with TS_AUTH_KEY when env var was setup. The root cause was a timing issue:

• Build Phase: Feature installs and entrypoint runs (secrets NOT available)
• Runtime Phase: GitHub Codespaces injects secrets (too late for entrypoint)

This meant TS_AUTH_KEY was undefined when the authentication logic ran.
Changes

• Added postCreateCommand to run auth script
• Removed auth logic from entrypoint
• Created new script for authentication

fixes: #53