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

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

Note: Manual ACP session cleanup testing was not performed.

[github-actions]

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[github-actions]

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[sauerdaniel]

Rebased onto latest dev and resolved merge conflicts.\n\nChanges adapted to the current architecture:\n- Added delete() and clear() methods to ACPSessionManager\n- Added dispose() method to Agent (aborts global event stream, clears sessions)\n- Added closeSession() method for per-session cleanup\n- Added handler for session.deleted event to auto-cleanup sessions\n- Removed per-session abortController from types (using global eventAbort pattern)\n\nAll typechecks pass. Ready for review.