pivot to Python: replace Kotlin/JVM with stdlib zipapp

Reasons stacked up:
  - AV: unsigned JARs that auto-download binaries + upload files trigger
    Windows Defender false-positives more often than Python scripts
    invoked by code-signed python.exe.
  - Qt UI option: PySide6 opens a path to a real Qt UI (matching Prism's
    look) if needed later. JVM Qt bindings are abandoned.
  - frazclient already needs Python; inlining as 'import cloud_sync' is
    zero overhead vs the launcher always shelling out to java.

Implementation:
  - cloud_sync package: cli.py (argparse), creds.py, scope.py,
    restic.py (binary discovery + auto-download + sha256 verify),
    sync.py (pull/push subprocess restic).
  - pyproject.toml with hatchling backend; pip-installable.
  - Makefile builds cloud-sync.pyz via python -m zipapp (~53 KB).
  - 33 pytest tests, stdlib only on runtime.
  - CI workflow runs pytest matrix (3.10/3.11/3.12) + builds pyz.
  - DESIGN.md + README.md updated to reflect Python.

E2E verified against local restic-rest-server:
  pull empty → push initial → rm -rf local → pull restores → modify+push
  creates second snapshot → client forget --prune blocked by --append-only.

Throws away ~565 LOC of Kotlin (and 18 jar tests) committed earlier in
this same session. Net result is ~250 LOC Python + 33 tests = smaller
and more aligned with the rest of the stack.

tests/test_cli.py

@@ -0,0 +1,75 @@
+"""CLI parsing tests — argv → (subcommand, Args)."""
+
+from __future__ import annotations
+
+import pytest
+
+from cloud_sync.cli import Args, parse
+
+
+def test_parses_pull_with_required_url() -> None:
+    cmd, args = parse(["pull", "--url=https://cloud.tm.center"])
+    assert cmd == "pull"
+    assert isinstance(args, Args)
+    assert args.url == "https://cloud.tm.center"
+    assert args.allow_download is True
+    assert args.headless is False
+
+
+def test_default_token_file_under_pack_folder() -> None:
+    _, args = parse(["pull", "--url=https://x", "--pack-folder=/tmp/inst"])
+    assert args.token_file.as_posix().endswith(".cloud-sync/token")
+    assert "/tmp/inst" in args.token_file.as_posix()
+
+
+def test_custom_token_file_overrides_default() -> None:
+    _, args = parse(
+        ["pull", "--url=https://x", "--pack-folder=/tmp/inst",
+         "--token-file=/etc/cloud-creds"]
+    )
+    assert args.token_file.as_posix() == "/etc/cloud-creds"
+
+
+def test_inline_and_space_separated_both_work() -> None:
+    _, a1 = parse(["pull", "--url=https://x", "--pack-folder=/srv"])
+    _, a2 = parse(["pull", "--url", "https://x", "--pack-folder", "/srv"])
+    assert a1.url == a2.url
+    assert a1.pack_folder == a2.pack_folder
+
+
+def test_no_gui_flag() -> None:
+    _, a = parse(["push", "--url=https://x", "-g"])
+    assert a.headless is True
+    _, b = parse(["push", "--url=https://x", "--no-gui"])
+    assert b.headless is True
+
+
+def test_no_download_flag() -> None:
+    _, a = parse(["push", "--url=https://x", "--no-download"])
+    assert a.allow_download is False
+
+
+def test_restic_binary_override() -> None:
+    _, a = parse(["push", "--url=https://x", "--restic-binary=/opt/restic"])
+    assert a.restic_binary is not None
+    assert a.restic_binary.as_posix() == "/opt/restic"
+
+
+def test_missing_url_exits() -> None:
+    with pytest.raises(SystemExit):
+        parse(["pull"])
+
+
+def test_missing_subcommand_exits() -> None:
+    with pytest.raises(SystemExit):
+        parse([])
+
+
+def test_unknown_subcommand_exits() -> None:
+    with pytest.raises(SystemExit):
+        parse(["bogus", "--url=https://x"])
+
+
+def test_pack_folder_is_resolved_to_absolute() -> None:
+    _, a = parse(["pull", "--url=https://x", "--pack-folder=."])
+    assert a.pack_folder.is_absolute()