Register() previously trusted a self-declared id and self-chosen token
from any caller, and let a second caller silently hijack an existing
worker id by re-registering it with a different token. Adds an optional
pre-shared AdmitToken (ORCHESTRA_FEDERATION_ADMIT_TOKEN) and requires a
same-id re-registration to present the existing worker's own token.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1rkJ2hBMybnJctPbcy4tT
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