feat: add ComputeSDK example & documentation

[makyinmars]

 Summary

Issue: #26

 - Add ComputeSDK example (examples/computesdk/) demonstrating how to run sandbox-agent using ComputeSDK's provider-agnostic sandbox API
 - Add deployment documentation (docs/deploy/computesdk.mdx) with TypeScript examples, provider configuration, and authentication guide
 - Supports auto-detection from 6 providers: E2B, Daytona, Vercel, Modal, Blaxel, CodeSandbox
 - Tested end-to-end with E2B and Vercel providers

 What's included

 Example code:

 - examples/computesdk/src/computesdk.ts - Setup function that auto-detects provider from environment, creates a sandbox, installs agents, and starts the server
 - examples/computesdk/tests/computesdk.test.ts - Integration test that verifies health endpoint (skipped when no ComputeSDK or provider credentials)

 Documentation:

 - Prerequisites (COMPUTESDK_API_KEY + any provider API key)
 - Complete TypeScript example showing provider detection, sandbox creation, agent installation, and streaming events
 - Provider environment variable reference table (E2B, Daytona, Vercel, Modal, Blaxel, CodeSandbox)
 - Explicit provider selection via COMPUTESDK_PROVIDER env var
 - Links to direct provider examples for users who don't want the ComputeSDK gateway

 Test plan

 - [x] Manually tested with E2B provider
 - [x] Manually tested with Vercel provider
 - [x] Verified sandbox creation and sandbox-agent installation
 - [x] Verified health endpoint responds correctly
 - [x] Test suite runs (skips when no credentials)