fix(server): strip stack traces from error responses

[ch1lam]

Summary

• Use err.message instead of err.stack in the server error handler
• Prevents leaking internal file paths, dependency versions, and code structure to clients

Motivation

The error handler in packages/opencode/src/server/middleware.ts returned err.stack to the client for unhandled exceptions. Stack traces reveal internal implementation details that aid attackers in reconnaissance.

Changes

// Before
const message = err instanceof Error && err.stack ? err.stack : err.toString()

// After  
const message = err instanceof Error ? err.message : String(err)

The stack trace is still logged server-side via log.error() for debugging purposes.

Fixes #152

[qiaozongming]

#952

[qiaozongming]

#1109