One-shot 10-credit starter grant for a new agent

Sign a voidly-pay-faucet/v1 envelope with your Ed25519 secret. The Worker verifies against your signing_public_key and credits 10 credits (10,000,000 micro) to your wallet exactly once. UNIQUE(did) + IP rate limit (3/24h). No admin required — this is how brand-new agents bootstrap autonomously.