fix(windows): terminal drain and cwd path conversion for native Windows

Two fixes for the local terminal backend on Windows (Git Bash):

1. `_drain()` in base.py: `select.select()` only works on sockets on
   Windows, not pipe file descriptors. On Windows, use blocking
   `os.read()` in the daemon thread instead. EOF arrives promptly
   when bash exits, so this is safe.

2. `_run_bash()` in local.py: When `self.cwd` is updated from `pwd`
   output, it contains Git Bash-style paths (`/c/Users/...`).
   `subprocess.Popen(cwd=...)` needs a native Windows path
   (`C:\Users\...`). Added a conversion before Popen.

Without these fixes, all terminal() calls on Windows return empty
output (exit code 126), and cwd tracking breaks.

Tested on Windows 11 with Git for Windows + Python 3.13.

Fixes #14638

tools/environments/base.py

@@ -489,6 +489,26 @@ class BaseEnvironment(ABC):
 
         def _drain():
             fd = proc.stdout.fileno()
+            # select.select does NOT work on pipe fds on Windows (only sockets).
+            # Use blocking os.read in a daemon thread instead — safe because
+            # EOF arrives promptly when bash exits.
+            if os.name == "nt":
+                try:
+                    while True:
+                        chunk = os.read(fd, 4096)
+                        if not chunk:
+                            break
+                        output_chunks.append(decoder.decode(chunk))
+                except (ValueError, OSError):
+                    pass
+                finally:
+                    try:
+                        tail = decoder.decode(b"", final=True)
+                        if tail:
+                            output_chunks.append(tail)
+                    except Exception:
+                        pass
+                return
             idle_after_exit = 0
             try:
                 while True: