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.

cloud_sync/restic.py

@@ -0,0 +1,223 @@
+"""restic binary discovery + auto-download + invocation.
+
+Discovery order:
+  1. ``--restic-binary <path>`` (explicit override)
+  2. ``<pack-folder>/.cloud-sync/restic-<RESTIC_VERSION>`` (pinned cached copy)
+  3. ``$PATH`` (only if version matches RESTIC_VERSION)
+  4. download from GitHub releases (unless ``--no-download``)
+
+The version is pinned because repos written by one restic version can have
+features another version can't read. Cache the pinned binary per-instance
+so deleting the instance dir wipes everything cloud-sync owns.
+"""
+
+from __future__ import annotations
+
+import bz2
+import hashlib
+import os
+import platform
+import re
+import shutil
+import stat
+import subprocess
+import sys
+import tempfile
+import urllib.request
+import zipfile
+from dataclasses import dataclass
+from pathlib import Path
+
+from .cli import Args
+
+
+RESTIC_VERSION = "0.18.0"
+RELEASE_TAG = f"v{RESTIC_VERSION}"
+
+_HTTP_TIMEOUT = 30
+
+
+@dataclass(frozen=True)
+class Platform:
+    os_tag: str       # linux, darwin, windows
+    arch_tag: str     # amd64, arm64
+    is_windows: bool
+
+
+def _detect_platform() -> Platform:
+    name = platform.system().lower()
+    arch_raw = platform.machine().lower()
+    if name.startswith("linux"):
+        os_tag = "linux"
+    elif name.startswith("darwin"):
+        os_tag = "darwin"
+    elif name.startswith("windows"):
+        os_tag = "windows"
+    else:
+        raise RuntimeError(f"unsupported platform: {name}")
+
+    if arch_raw in ("amd64", "x86_64"):
+        arch_tag = "amd64"
+    elif arch_raw in ("aarch64", "arm64"):
+        arch_tag = "arm64"
+    else:
+        raise RuntimeError(f"unsupported architecture: {arch_raw}")
+    return Platform(os_tag, arch_tag, os_tag == "windows")
+
+
+def _binary_filename(plat: Platform) -> str:
+    suffix = ".exe" if plat.is_windows else ""
+    return f"restic-{RESTIC_VERSION}{suffix}"
+
+
+def resolve_binary(args: Args) -> Path:
+    """Return a usable restic binary path, downloading if needed."""
+    if args.restic_binary is not None:
+        if not args.restic_binary.exists():
+            raise FileNotFoundError(
+                f"--restic-binary path does not exist: {args.restic_binary}"
+            )
+        return args.restic_binary
+
+    plat = _detect_platform()
+    cache_dir = args.pack_folder / ".cloud-sync"
+    cached = cache_dir / _binary_filename(plat)
+    if cached.exists() and os.access(cached, os.X_OK):
+        return cached
+
+    # Try $PATH only when version matches exactly
+    system = _find_system_binary_matching_version()
+    if system is not None:
+        return system
+
+    if not args.allow_download:
+        raise RuntimeError(
+            f"no usable restic binary at {cached} or on $PATH, "
+            f"and --no-download disabled auto-fetch"
+        )
+
+    cache_dir.mkdir(parents=True, exist_ok=True)
+    _download_restic_to(cached, plat)
+    return cached
+
+
+def run(
+    binary: Path,
+    args: list[str],
+    env: dict[str, str] | None = None,
+    cwd: Path | None = None,
+    timeout: int = 900,
+) -> tuple[int, str]:
+    """Run restic. Inherits stderr to caller's terminal for live progress.
+    Returns (returncode, captured_stdout)."""
+    merged_env = dict(os.environ)
+    if env:
+        merged_env.update(env)
+    p = subprocess.run(  # noqa: S603 — controlled invocation
+        [str(binary), *args],
+        cwd=str(cwd) if cwd else None,
+        env=merged_env,
+        stdout=subprocess.PIPE,
+        stderr=sys.stderr,
+        text=True,
+        timeout=timeout,
+        check=False,
+    )
+    return p.returncode, p.stdout
+
+
+# ---------------------------------------------------------------------------
+# discovery + download helpers
+# ---------------------------------------------------------------------------
+
+
+def _find_system_binary_matching_version() -> Path | None:
+    name = "restic.exe" if _detect_platform().is_windows else "restic"
+    found = shutil.which(name)
+    if not found:
+        return None
+    path = Path(found)
+    return path if _binary_version(path) == RESTIC_VERSION else None
+
+
+def _binary_version(path: Path) -> str | None:
+    try:
+        out = subprocess.run(  # noqa: S603
+            [str(path), "version"],
+            capture_output=True, text=True, timeout=5, check=False,
+        )
+    except (subprocess.SubprocessError, OSError):
+        return None
+    text = out.stdout + out.stderr
+    m = re.search(r"restic\s+(\d+\.\d+\.\d+)", text)
+    return m.group(1) if m else None
+
+
+def _download_restic_to(target: Path, plat: Platform) -> None:
+    ext = "zip" if plat.is_windows else "bz2"
+    asset = f"restic_{RESTIC_VERSION}_{plat.os_tag}_{plat.arch_tag}.{ext}"
+    asset_url = (
+        f"https://github.com/restic/restic/releases/download/"
+        f"{RELEASE_TAG}/{asset}"
+    )
+    sums_url = (
+        f"https://github.com/restic/restic/releases/download/"
+        f"{RELEASE_TAG}/SHA256SUMS"
+    )
+    print(
+        f"cloud-sync: downloading restic {RESTIC_VERSION} from {asset_url}",
+        file=sys.stderr,
+    )
+    with tempfile.NamedTemporaryFile(suffix=f".{ext}", delete=False) as tmp:
+        tmp_path = Path(tmp.name)
+    try:
+        with urllib.request.urlopen(asset_url, timeout=_HTTP_TIMEOUT) as r:
+            tmp_path.write_bytes(r.read())
+        expected = _expected_sha(sums_url, asset)
+        actual = _sha256_file(tmp_path)
+        if expected.lower() != actual.lower():
+            raise RuntimeError(
+                f"restic download sha mismatch: expected {expected}, got {actual}"
+            )
+        _decompress_into(tmp_path, target, ext)
+        if not plat.is_windows:
+            target.chmod(target.stat().st_mode | stat.S_IXUSR | stat.S_IRUSR | stat.S_IWUSR)
+    finally:
+        tmp_path.unlink(missing_ok=True)
+
+
+def _expected_sha(sums_url: str, asset: str) -> str:
+    with urllib.request.urlopen(sums_url, timeout=_HTTP_TIMEOUT) as r:
+        body = r.read().decode("utf-8")
+    for line in body.splitlines():
+        line = line.strip()
+        if line.endswith(asset):
+            return line.split()[0]
+    raise RuntimeError(f"restic SHA256SUMS missing entry for {asset}")
+
+
+def _sha256_file(path: Path) -> str:
+    h = hashlib.sha256()
+    with path.open("rb") as f:
+        for chunk in iter(lambda: f.read(65536), b""):
+            h.update(chunk)
+    return h.hexdigest()
+
+
+def _decompress_into(archive: Path, target: Path, ext: str) -> None:
+    target.parent.mkdir(parents=True, exist_ok=True)
+    if ext == "bz2":
+        with bz2.open(archive, "rb") as src, target.open("wb") as dst:
+            shutil.copyfileobj(src, dst)
+    elif ext == "zip":
+        with zipfile.ZipFile(archive) as zf:
+            inner = next(
+                (n for n in zf.namelist() if n.endswith("restic.exe")),
+                None,
+            )
+            if inner is None:
+                raise RuntimeError("restic.exe not found in downloaded zip")
+            with zf.open(inner) as src, target.open("wb") as dst:
+                shutil.copyfileobj(src, dst)
+    else:
+        raise RuntimeError(f"unsupported archive ext: {ext}")