lib.FileContext: Add readdir() method

FileContext offers get(), put(), unlink(), mkdir(), and stat(), but no way
to list the entries of a directory, so consumers that need the names of the
entries under a path in a context have no portable means to obtain them.

Add readdir(), returning Iterable[str], following the _readdir() ->
readdir() pattern used by the other operations: the public method chroots
the path via _chroot() and delegates to the _readdir() hook, which raises
NotImplementedError by default.

Implement _readdir() in the context families:

- Local._readdir(): os.listdir()
- ExecContext._readdir(): runs ls -1A and splits the output into one entry
  per line; inherited by SSHClient and the other command-based contexts
- Curl._readdir(): raises OSError with errno.ENOSYS

Add unit tests covering Local (listing, hidden entries, missing directory)
and Curl (ENOSYS).

Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.87.1
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/ec/Local.py

@@ -7,7 +7,7 @@ import pwd
 import sys
 
 from functools import cache
-from typing import TYPE_CHECKING, override
+from typing import TYPE_CHECKING, Iterable, override
 
 from ..base import Result, StatResult
 from ..ExecContext import ExecContext as Base
@@ -206,3 +206,7 @@ class Local(Base):
         if (not follow_symlinks) and os.path.islink(path):
             return False
         return os.path.isdir(path)
+
+    @override
+    async def _readdir(self, path: str) -> Iterable[str]:
+        return os.listdir(path)