Memory leak: Multiple opencode processes accumulate over days and don't terminate on session exit #14199

When using opencode interactively, multiple opencode processes accumulate over time and don't terminate properly when sessions end. This leads to multiple processes running simultaneously (5+ observed), each consuming 300-1000MB, with processes dating back 3-5 days. Total memory consumption reached 3.35GB across 5 processes, contributing to system freeze on Ubuntu server (94.44% RAM used, OOM crash).

Steps to reproduce:

1. Start opencode session
2. Use it for some time
3. Exit session normally
4. Start new session in different project
5. Repeat over several days
6. Check: ps aux | grep opencode

Actual: Multiple opencode processes remain after sessions end (PID examples from 3-5 days old)

Expected: Process should terminate on session exit, releasing memory and file descriptors.

Environment: opencode 1.2.x on macOS and Linux (arm64/x86_64)

Root cause: Background processes not killed on session exit, event listeners not cleaned up, improper signal handling.

Related issues: #13477, #13796, #13778, #13777

Proposed fixes: Proper session tracking and child process termination on exit, auto-cleanup of stale processes (>24h), improved process lifecycle in opencode serve, --cleanup flag.