fix: Enforce active session cap in UnifiedExecManager #146

Summary

MAX_SESSIONS is defined as 64, but active sessions are not capped. Cleanup only removes exited sessions when total size exceeds the limit.

• Affected code: /Users/mcell/Desktop/workspace/memo-cli/packages/tools/src/tools/exec_runtime.ts:13
• Cleanup logic: /Users/mcell/Desktop/workspace/memo-cli/packages/tools/src/tools/exec_runtime.ts:160

Why this matters

A model can create many long-running commands and exceed intended limits, causing uncontrolled process count and memory/FD pressure.

Repro

Run exec_command many times with a long-running command (sleep 5) and yield_time_ms: 0.

Local result from 70 calls:

• { running: 70, errors: 0 }

This exceeds MAX_SESSIONS=64 with no rejection.

Expected

New sessions should be rejected (or oldest should be terminated/evicted) once active session limit is reached.

Actual

No limit is enforced for running sessions.

Suggested fix

• Track active (non-exited) session count and reject or evict above limit.
• Optionally add idle timeout/TTL cleanup policy.
• Add regression tests for cap enforcement.

Requested implementer

Please let memo handle this fix.