refactor(environments): extract _ThreadedProcessHandle base class

Eliminates ~50 lines of duplicated pipe+thread+poll boilerplate between
_ModalProcessHandle and _DaytonaProcessHandle. Both now use closures
passed to the shared _ThreadedProcessHandle in base.py.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/tools/test_threaded_process_handle.py

@@ -0,0 +1,63 @@
+"""Unit tests for the _ThreadedProcessHandle adapter in base.py."""
+
+import threading
+
+import pytest
+
+
+def test_successful_execution():
+    """exec_fn returns (output, 0) -> returncode==0 and stdout reads output."""
+    from tools.environments.base import _ThreadedProcessHandle
+
+    handle = _ThreadedProcessHandle(lambda: ("hello output", 0))
+    handle.wait()
+    assert handle.returncode == 0
+    text = handle.stdout.read()
+    assert text == "hello output"
+    handle.stdout.close()
+
+
+def test_nonzero_exit_code():
+    """exec_fn returns (output, 42) -> returncode==42."""
+    from tools.environments.base import _ThreadedProcessHandle
+
+    handle = _ThreadedProcessHandle(lambda: ("error output", 42))
+    handle.wait()
+    assert handle.returncode == 42
+    text = handle.stdout.read()
+    assert text == "error output"
+    handle.stdout.close()
+
+
+def test_exception_returns_rc1():
+    """exec_fn raises RuntimeError -> returncode==1 and error message in stdout."""
+    from tools.environments.base import _ThreadedProcessHandle
+
+    def failing_fn():
+        raise RuntimeError("boom")
+
+    handle = _ThreadedProcessHandle(failing_fn)
+    handle.wait()
+    assert handle.returncode == 1
+    text = handle.stdout.read()
+    assert "boom" in text
+    handle.stdout.close()
+
+
+def test_poll_returns_none_while_running():
+    """poll() returns None before exec_fn completes."""
+    from tools.environments.base import _ThreadedProcessHandle
+
+    barrier = threading.Event()
+
+    def blocking_fn():
+        barrier.wait(timeout=5)
+        return ("done", 0)
+
+    handle = _ThreadedProcessHandle(blocking_fn)
+    assert handle.poll() is None
+    barrier.set()
+    handle.wait()
+    assert handle.poll() == 0
+    handle.stdout.read()
+    handle.stdout.close()