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/FileContext.py

@@ -4,7 +4,7 @@ import abc
 
 from enum import Enum, auto
 from functools import cached_property
-from typing import TYPE_CHECKING, Any, override
+from typing import TYPE_CHECKING, Any, Iterable, override
 
 from .log import DEBUG, ERR, log
 from .ProcFilter import ProcPipeline
@@ -301,6 +301,14 @@ class FileContext(abc.ABC):
     async def is_dir(self, path: str, follow_symlinks: bool = True) -> bool:
         return await self._is_dir(self._chroot(path), follow_symlinks = follow_symlinks)
 
+    async def _readdir(self, path: str) -> Iterable[str]:
+        raise NotImplementedError(
+            f'{self.log_name}: readdir("{path}") is not implemented'
+        )
+
+    async def readdir(self, path: str) -> Iterable[str]:
+        return await self._readdir(self._chroot(path))
+
     @classmethod
     def create(cls, uri: str | Uri, *args: Any, **kwargs: Any) -> 'FileContext':
         uri = Uri.pimp(uri)