fix: add signal handlers to prevent orphaned processes

[pedropombeiro]

Summary

• Add global signal handlers (SIGTERM, SIGINT, SIGHUP) for graceful shutdown
• Calls Instance.disposeAll() to clean up LSP servers, MCP clients, PTY sessions, workers
• Uses 5-second timeout before force exit
• Uses standard Unix exit codes (128 + signal number)

Fixes

• Fixes OpenCode leaves an orphaned process when it itself is killed #11527 - OpenCode leaves an orphaned process when killed
• Fixes [BUG] Opencode process survives terminal close — missing SIGHUP handler causes orphaned process accumulation #12767 - Missing SIGHUP handler causes orphaned process accumulation
• Fixes Memory Leak: Orphaned processes when terminal closes without explicit exit #10563 - Orphaned processes when terminal closes without explicit exit

[github-actions]

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

Potential Duplicate PRs Found

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

   • Directly related: Addresses SIGHUP handler specifically for orphaned processes when terminal closes
   • Similar scope: Signal handling for graceful shutdown

2. PR fix: prevent orphaned worker process when killed #11603 - fix: prevent orphaned worker process when killed

   • Related: Tackles orphaned process prevention from a different angle (worker processes)
   • May overlap with the disposeAll() cleanup logic

3. PR fix: force kill MCP server processes on dispose to prevent orphan processes #7424 - fix: force kill MCP server processes on dispose to prevent orphan processes

   • Related: Process cleanup and orphan prevention for MCP servers
   • May be superseded or combined with this PR's broader disposeAll() approach

The PR #12718 is particularly relevant as it appears to address part of the same problem (SIGHUP handling) that this PR is trying to solve more comprehensively with global signal handlers.