feat: implement agent payment protocol with stream negotiation, on-chain micropayments, and CortexAgentSDK

[Yunusabdul38]

Description

This pull request implements the payment protocol layer enabling autonomous agents to discover assets, negotiate rates, manage payment streams, and execute batch settlements programmatically.

Changes

Smart Contract (Rust)

• Added batch_settle and get_claimable_batch functions to the micropayments contract.
• Registered unit tests and resolved a withdrawal bug in withdraw.

Database

• Added calls_remaining, calls_used, and price_per_call columns to the streams table.
• Added row locking and call counter update functions in streamRepository.js.

Backend Layer

• QuoteManager.js: HMAC-SHA256 signed price quotes with TTL memory caching.
• StreamNegotiator.js: Off-chain rate negotiation and JWT stream token generation.
• MeteringEngine.js: Atomic call counter decrementing using database transactions.
• BatchSettler.js: Periodic bulk withdrawals on streams with 25 or more calls.
• StreamMonitor.js: SSE registration and warning alerts at 10% remaining calls.
• Exposed new API routes under /api/v1/protocol/*.

Agent SDK

• CortexAgentSDK.js client covering the end-to-end lifecycle.
• Complete TypeScript definitions (CortexAgentSDK.d.ts) and user documentation (README.md).

Verification

• Smart contract unit tests (cargo test) pass.
• Backend linter and Jest tests (npm run lint & npm test) pass.

Closes #14

[vestor-dev]

lgtm