feat: add x402 payment protocol support via AgentKit (#252)

- Add wallet management (init, balance, history) via CLI and agent tools
- Add x402 payment tools (fetch_payment_info, paid_request, wallet_info, wallet_history)
- Use @x402/fetch + @x402/evm directly to avoid Privy browser-env conflicts
- Add /wallet TUI modal for payment settings (enabled, chain, auto-approve)
- Add inline payment approval panel when auto-approve is disabled
- Add JSONL audit trail at ~/.grok/payment_log.jsonl
- Add payment settings in ~/.grok/user-settings.json
- Support Base mainnet and Base Sepolia chains