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>

test/unit/python/jw/pkg/lib/ec/Curl/Makefile

@@ -0,0 +1,7 @@
+TOPDIR = ../../../../../../../..
+
+include $(TOPDIR)/make/proj.mk
+include $(JWBDIR)/make/py-run.mk
+
+all:
+test: run

test/unit/python/jw/pkg/lib/ec/Curl/test.py

@@ -0,0 +1,15 @@
+import asyncio
+import errno
+
+from jw.pkg.lib.ec.Curl import Curl
+
+async def run_tests() -> None:
+    curl = Curl('https://example.com/')
+    try:
+        await curl.readdir('some/path')
+        raise AssertionError('expected OSError with errno ENOSYS')
+    except OSError as e:
+        assert e.errno == errno.ENOSYS, e.errno
+
+asyncio.run(run_tests())
+print('All Curl readdir tests passed')