refactor!: port shell scripts to Python package

Bash scripts were untestable and duplicated device/LUKS/mount logic;
the lim/ package centralizes it behind one subprocess wrapper and a
YAML image catalog (single point of truth).

BREAKING CHANGE: scripts/*.sh removed. Use `lim --type <cmd>`; new
types mount/umount/single-boot/raid1-boot/lock/unlock/import/export
replace direct script calls. --extra is deprecated and ignored.

- distributions.yml + lim/catalog.py hold the image catalog (PyYAML)
- pytest suite: 102 tests with mocked subprocess (tests/unit) and a
  250-line max file-length guard (tests/lint)
- ruff strict (select ALL), GitHub Actions CI, Dependabot; Travis gone
- Makefile: install (symlink ~/.local/bin/lim) and test targets
- fixes over bash: SUDO_USER-aware chown, mmcblk/nvme partition paths,
  sha512 checksum support, whole-pipeline failure detection, blkid
  UUID fallback for pre-mounted images, conditional fstab seeding for
  PARTUUID/LABEL images, clean errors for missing binaries

lim/fsutil.py

@@ -0,0 +1,32 @@
+"""Small file manipulation helpers."""
+
+from pathlib import Path
+
+from lim import ui
+from lim.errors import LimError
+
+
+def replace_in_file(search: str, replace: str, path: str | Path) -> None:
+    """Replace every literal occurrence of ``search``; fail when absent."""
+    path = Path(path)
+    text = path.read_text()
+    new_text = text.replace(search, replace)
+    if new_text == text:
+        raise LimError(f"Search string '{search}' not found in {path}.")
+    path.write_text(new_text)
+
+
+def ensure_line_in_file(line: str, path: str | Path) -> bool:
+    """Append ``line`` unless already present. Returns True when appended."""
+    path = Path(path)
+    content = path.read_text() if path.exists() else ""
+    if line in content.splitlines():
+        ui.warning(f"File {path} already contains the following entry:")
+        print(line)
+        ui.info("Skipped.")
+        return False
+    with path.open("a") as handle:
+        if content and not content.endswith("\n"):
+            handle.write("\n")
+        handle.write(line + "\n")
+    return True