refactor(util): move expand_tilde to top-level claude_bottle/util.py

_expand_tilde was a path-string helper on DockerBottleBackend but
nothing about it was Docker-specific — any future backend reading
host paths from manifest entries would want it. Lift to
claude_bottle/util.py (sibling of log.py / manifest.py) as a public
expand_tilde() function. Docker-specific primitives stay in
claude_bottle/backend/docker/util.py.

claude_bottle/util.py

@@ -0,0 +1,18 @@
+"""Cross-cutting utility helpers used by multiple modules.
+
+Top-level (i.e. backend-agnostic) — Docker-specific helpers live in
+claude_bottle/backend/docker/util.py."""
+
+from __future__ import annotations
+
+import os
+
+
+def expand_tilde(path: str) -> str:
+    """Expand a leading '~' to $HOME. Leaves paths without a leading
+    tilde unchanged. Falls back to the empty string if $HOME is unset
+    (callers should already have checked HOME if they care)."""
+    if path.startswith("~"):
+        home = os.environ.get("HOME", "")
+        return home + path[1:]
+    return path