fix(tui): handle SIGINT and SIGTERM for graceful shutdown

[rektide]

Issue for this PR

Closes #24658

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Handles SIGTERM and SIGINT the same way SIGHUP is handled, having the UI trigger a full cleanup.

How did you verify your code works?

Ran it locally.

Screenshots / recordings

  Session   opencode-sample-session
  Continue  opencode -s ses_22f9e3302ffe63raHGWJ04o92l

(Successful exit!)

Checklist

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

If you do not follow this template your PR will be automatically rejected. 👍

[github-actions]

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

Based on my search, I found several related PRs that address signal handling and graceful shutdown:

Potentially Related PRs:

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

   • Directly relevant as this PR mentions handling SIGHUP (which is referenced in PR fix(tui): handle SIGINT and SIGTERM for graceful shutdown #24659)

2. #24297 - fix(opencode): resolve heap unlimited + orphan processes on Linux

   • Related to process cleanup and signal handling

3. #20006 - fix: clean up orphaned processes and LSP state on shutdown

   • Related to shutdown cleanup logic

4. #9866 - fix(serve): add signal handlers for graceful shutdown on Windows

   • Similar signal handler implementation for graceful shutdown

These PRs address similar concerns around signal handling (SIGTERM, SIGINT, SIGHUP) and graceful shutdown. The most relevant appears to be #12718 which specifically handles SIGHUP for process cleanup - exactly the mechanism PR #24659 aims to extend to SIGTERM and SIGINT.