fix(session): add message limit warning to prevent memory leaks in long-running sessions

[walker83]

PR: Add session message limit warning to prevent memory leaks

Summary

This PR adds a warning log when session message count exceeds 1000 messages, helping users identify potential memory leak issues in long-running sessions.

Problem

As reported in #1221, long-running sessions (>200k tokens) become extremely slow and eventually crash. Root cause analysis shows that:

1. Message history grows without bounds
2. Each message contains full content, metadata, and token stats
3. No early warning system exists to alert users

Solution

Add MAX_MESSAGES_PER_SESSION = 1000 constant and log a warning when exceeded:

if (input.messages.length > MAX_MESSAGES_PER_SESSION) {
  log.warn("session.message-limit-exceeded", {
    sessionID: input.sessionID,
    messageCount: input.messages.length,
    limit: MAX_MESSAGES_PER_SESSION,
    suggestion: "Consider starting a new session or running /compact",
  })
}

Changes

• packages/opencode/src/session/compaction.ts:
  • Add MAX_MESSAGES_PER_SESSION export constant
  • Add warning log in processCompaction function

Testing

1. Start a new session
2. Send messages until count exceeds 1000
3. Check logs for warning: session.message-limit-exceeded

Related Issues

• Fixes Long running sessions cause lag and an eventual crash #1221 (Long running sessions cause lag and crash)
• Related: 使用ultraspeed模型，长时间思考会卡死，并且会偶发性会出现tui乱码 #813 (长时间思考会卡死), 超大内存占用 #680 (超大内存占用), 疑似内存泄露 #300 (疑似内存泄露)

Checklist

• Code follows project style guidelines
• Added inline comments explaining the change
• Linked to related issues
• Commit message follows conventional commit format

Notes

This is a diagnostic fix that adds visibility into the memory issue. Future PRs could:

1. Implement automatic session archival when limit is exceeded
2. Add memory usage monitoring
3. Implement message pagination for large sessions