Memory leak: ACP session event streams never aborted

[sauerdaniel]

Problem

In the ACP (Agent Communication Protocol) module, sessions and their event streams are not properly cleaned up:

1. Session Map Growth - Sessions are stored in a Map but never removed when completed
2. Event Stream Leaks - Event streams created via session.stream() are never aborted when sessions end
3. No Disposal Mechanism - No dispose() method exists to clean up session resources

Code Location

• packages/opencode/src/acp/agent.ts
• packages/opencode/src/acp/session.ts
• packages/opencode/src/acp/types.ts

Impact

• Memory grows with each session that completes
• Event stream listeners accumulate
• No way to explicitly clean up session resources

Relates to #5363

Expected Behavior

• Sessions should be removed from Map when completed
• Event streams should be aborted when session ends
• A dispose() method should be available for explicit cleanup