fix(acp): Fix memory leak in session event streams

[sauerdaniel]

Summary

Add proper cleanup for ACP (Agent Communication Protocol) sessions to prevent resource leaks from orphaned sessions.

Fixes #9154 Relates to #5363

Problem

When ACP sessions are created:

1. Sessions may not be properly cleaned up when the parent process ends
2. Event listeners may not be removed
3. Orphaned sessions can accumulate

Solution

Add proper session lifecycle management:

• Track active sessions
• Clean up sessions on process exit
• Remove event listeners properly

Changes

• packages/opencode/src/acp/session.ts - Add session cleanup logic

Testing

• [x] TypeScript compilation passes (bun turbo typecheck)
• [x] Unit tests pass (725 tests, 0 failures)

Note: Manual ACP session cleanup testing was not performed.