fix: use fixed port 1248 and SessionStart hook

Fixed port eliminates the need for dynamic ANTHROPIC_BASE_URL
injection. Users set env.ANTHROPIC_BASE_URL in settings.json once.
The SessionStart hook starts the proxy on port 1248 and the
SessionEnd hook kills it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>