[subproc] add exit runtime code

[mr-chelyshkin]

📝 Description

Add exit_code from subprocess

Fix

• TaskError::Fail { reason, exit_code: Option<i32> }
• TaskError::Fatal { reason, exit_code: Option<i32> }
• Event::exit_code: Option<i32> + with_exit_code(i32) builder
• Actor propagates exit_code from TaskError into the corresponding TaskFailed / ActorDead events.

Tests

• TaskError::Fail/Fatal round-trip with exit_code.
• Event::with_exit_code populates the field.
• End-to-end: actor receiving TaskError::Fail { exit_code: Some(5) } emits TaskFailed { event.exit_code == Some(5) }.

🔄 Related Issues

Resolves #68

✅ Checklist

• Documentation updated (README, rustdoc, examples)
• Tests added/updated (if relevant)
• CI passes locally