feat: integrate subscriptions via tributary

[xeroc]

Integrates Tributary protocol for on-chain recurring subscriptions on Solana.

Adds 6 new CLI commands under zerion subscribe:

• create — create a recurring USDC payment to a recipient with configurable interval (daily through annually), auto-renew, max-renewals, and memo
• list — list all payment policies for a wallet, with status filtering and pretty table output
• show — display single policy details
• pause / resume — toggle policy status on-chain
• delete — permanently remove a policy

Supports all Tributary policy types (subscription, milestone, pay-as-you-go) for read operations. New dependencies: @tributary-so/sdk, bn.js. Includes SDK wrapper with local keystore signing, format helpers, and a zerion-tributary skill for agent discovery.

[xeroc]

This pull request comes as a result of my application to this:

https://superteam.fun/earn/listing/build-an-autonomous-onchain-agent-using-the-zerion-cli

Application Context:

Tributary is a production-grade recurring payments protocol on Solana that delivers Web2 subscription UX with Web3 transparency. It solves a critical gap in the Solana ecosystem: there is no native, composable infrastructure for subscriptions, milestone payments, or pay-as-you-go billing.