fix(opencode): resolve heap unlimited + orphan processes on Linux

[herjarsa]

Issue for this PR

Closes #15348

Type of change

• Bug fix

What does this PR do?

Fixes critical memory leak and orphan process issues on Linux:

1. Orphan processes: Added SIGTERM handler alongside existing SIGHUP handler in exit.tsx and hread.ts to ensure graceful shutdown when terminal closes or process receives termination signal.

2. Heap growth (37+ GB): Added Effect.ensuring to abort AbortController in prompt.ts execRead function. Previously, controllers were only aborted on interrupt, causing accumulation during normal operation.

3. Unbounded cache: Replaced unbounded Map with LRU cache (max 20 entries) in instance.ts to prevent unbounded growth of instance contexts.

How did you verify your code works?

• Ran �un typecheck in packages/opencode - passes with no errors
• Verified signal handlers are properly registered and cleaned up
• Confirmed AbortController is aborted in both interrupt and success paths
• LRU cache bounds instance retention to 20 entries max

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search results, I found potential related PRs:

Related PRs:

1. PR fix(cli): handle SIGHUP to prevent orphaned processes on terminal close #12718 - fix(cli): handle SIGHUP to prevent orphaned processes on terminal close

   • Related to the SIGTERM/SIGHUP handler work for graceful shutdown

2. PR fix: resolve multiple memory leaks causing unbounded growth #16695 - fix: resolve multiple memory leaks causing unbounded growth

   • Similar scope: addresses memory leaks with unbounded growth

3. PR fix(opencode): memory leaks in session handling #10392 - fix(opencode): memory leaks in session handling

   • Memory leak related work

4. PR feat(util): Add bounded LRU cache utility #9141 - feat(util): Add bounded LRU cache utility

   • Related to the LRU cache implementation mentioned in this PR

These PRs address similar issues (memory management, process cleanup, signal handling), but since PR #24297 (the current PR) is a comprehensive fix specifically addressing the orphan processes + heap growth on Linux, you may want to verify these related PRs are either closed/merged or confirm there's no overlapping work.