Fix #20: Enable immediate child process access for killing

[konard]

Summary

Enables users to access and kill child processes immediately without having to await the process result first, addressing issue #20.

The key issue was that process.child.kill('SIGTERM') was not accessible without first awaiting the process, making it impossible to kill long-running processes before they complete.

Changes

• Add child getter: Provides immediate access to child process for killing
• Virtual command support: Return proxy object with kill() method that delegates to ProcessRunner
• Real command support: Auto-start process when child is accessed
• Internal refactoring: Update references from this.child to this._child for internal use
• Bug fix: Fix kill() method to properly handle virtual commands

Usage

Both virtual and real commands now support immediate killing:

// Virtual command (uses proxy child)
const proc1 = $`sleep 10`;
proc1.child.kill('SIGTERM'); // ✅ Works immediately

// Real command (auto-starts and provides real child)  
const proc2 = $`/bin/sleep 10`;
proc2.child.kill('SIGTERM'); // ✅ Works immediately

Test plan

• ✅ Added comprehensive test suite in tests/child-access.test.mjs
• ✅ All existing tests pass (ctrl-c-basic.test.mjs, $.test.mjs, streaming-interfaces.test.mjs)
• ✅ Verified both virtual and real commands support killing
• ✅ Confirmed processes actually terminate with correct exit codes
• ✅ No breaking changes to existing API

🤖 Generated with Claude Code

---

Resolves #20