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/backend/docker/backend.py

@@ -21,6 +21,7 @@ from ... import pipelock
 from ...env_resolve import env_resolve
 from ...log import die, info
 from ...manifest import SshEntry
+from ...util import expand_tilde
 from .. import BottleBackend, BottleCleanupPlan, BottlePlan, BottleSpec
 from . import network as network_mod
 from . import util as docker_mod
@@ -360,7 +361,7 @@ class DockerBottleBackend(BottleBackend):
         the y/N so the user doesn't get prompted for a plan with a
         missing key."""
         for entry in entries:
-            key = self._expand_tilde(entry.IdentityFile)
+            key = expand_tilde(entry.IdentityFile)
             if not os.path.isfile(key):
                 die(f"ssh key file not found for host '{entry.Host}': {key}")
 
@@ -431,7 +432,7 @@ class DockerBottleBackend(BottleBackend):
         container_key_paths: list[str] = []
         for entry in bottle.ssh:
             name = entry.Host
-            key = self._expand_tilde(entry.IdentityFile)
+            key = expand_tilde(entry.IdentityFile)
             hostname = entry.Hostname
             user = entry.User
             port = entry.Port
@@ -540,12 +541,6 @@ class DockerBottleBackend(BottleBackend):
             check=True,
         )
 
-    def _expand_tilde(self, path: str) -> str:
-        if path.startswith("~"):
-            home = os.environ.get("HOME", "")
-            return home + path[1:]
-        return path
-
     def provision_git(self, plan: BottlePlan, target: str) -> None:
         """If --cwd was set and the host cwd has a .git directory, copy
         it into /home/node/workspace/.git and fix ownership. No-op

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