Send one transactional SMS message to a US phone number. Costs $0.03 USDC per message, paid automatically from the configured wallet on Base Sepolia (testnet during beta, will flip to Base mainnet at launch). Messages are sent from a Twilio toll-free number and automatically get 'Reply STOP to opt out' appended if not already present. CRITICAL — only call this tool when ALL of the following are true: (1) the user has explicitly asked to text someone, or has previously set up an automation that texts; (2) the recipient is the user themselves OR someone the user has confirmed has consented to receive texts from this automation; (3) the content is transactional or informational (order updates, alerts, reminders, 2FA, agent task completion notifications) — NOT marketing, promotion, or unsolicited outreach. If unsure whether the recipient consented, ASK the user before calling. Setting opt_in_attestation: true is the operator's (user's) legal attestation that consent was obtained — it is not a no-op flag. Use cases: 'text me when the long-running task finishes', 'send Mom an order confirmation for the gift I just bought her', 'remind me at 3pm to call the dentist by texting my cell'.