feat: add install command

Not wired yet

src/caelestia/utils/dots/deployer.py

@@ -0,0 +1,58 @@
+import shutil
+import tempfile
+from pathlib import Path
+
+
+class Deployer:
+    """Places files from the dots clone into their destinations."""
+
+    def place(self, src: Path, dest: Path) -> None:
+        """Place a whole entry (file or directory tree), replacing any existing dest."""
+
+        if src.is_dir():
+            self.place_dir(src, dest)
+        else:
+            self.place_file(src, dest)
+
+    def place_dir(self, src: Path, dest: Path) -> None:
+        """Place a directory tree recursively, replacing any existing dest."""
+
+        if dest.is_symlink() or dest.is_file():
+            self.remove(dest)
+
+        dest.mkdir(parents=True, exist_ok=True)
+        for path in src.rglob("*"):
+            if path.is_file():
+                self.place_file(path, dest / path.relative_to(src))
+            elif path.is_dir():
+                (dest / path.relative_to(src)).mkdir(parents=True, exist_ok=True)
+
+    def place_file(self, src: Path, dest: Path) -> None:
+        """Atomically place a single file, replacing any existing dest."""
+
+        if dest.is_dir() and not dest.is_symlink():
+            self.remove(dest)
+
+        dest.parent.mkdir(parents=True, exist_ok=True)
+        f = tempfile.NamedTemporaryFile(dir=dest.parent, delete=False)
+        f.close()
+        try:
+            shutil.copyfile(src, f.name)
+            shutil.copymode(src, f.name)
+            Path(f.name).replace(dest)
+        except BaseException:
+            Path(f.name).unlink()
+            raise
+
+    def write_new(self, src: Path, dest: Path) -> Path:
+        """Write the upstream version alongside dest as <dest>.new and return that path."""
+
+        new_path = dest.parent / f"{dest.name}.new"
+        self.place_file(src, new_path)
+        return new_path
+
+    def remove(self, path: Path) -> None:
+        if path.is_symlink() or path.is_file():
+            path.unlink()
+        elif path.is_dir():
+            shutil.rmtree(path)