refactor(util): split private helpers off DockerBottleBackend

New file claude_bottle/backend/util.py for cross-backend host-side
helpers:
  host_skill_dir(name) — resolves $HOME/.claude/skills/<name>

docker/util.py gains:
  docker_exec_root(container, argv) — `docker exec -u 0` wrapper used
    by SSH provisioning

DockerBottleBackend drops the two methods that wrapped these
(`_host_skill_dir`, `_docker_exec_root`) — they had no instance state
and just lived on the class for organizational reasons. Call sites
now use the imported functions directly.

claude_bottle/backend/docker/backend.py

@@ -24,6 +24,7 @@ from ...log import die, info
 from ...manifest import SshEntry
 from ...util import expand_tilde
 from .. import BottleBackend, BottleCleanupPlan, BottlePlan, BottleSpec
+from ..util import host_skill_dir
 from . import network as network_mod
 from . import util as docker_mod
 from .bottle import DockerBottle
@@ -307,19 +308,13 @@ class DockerBottleBackend(BottleBackend):
         user doesn't get a launch prompt for a plan that's already
         known to break."""
         for name in skills:
-            path = self._host_skill_dir(name)
+            path = host_skill_dir(name)
             if not os.path.isdir(path):
                 die(
                     f"skill '{name}' not found on host at {path}. "
                     f"Create it under ~/.claude/skills/, then re-run."
                 )
 
-    def _host_skill_dir(self, name: str) -> str:
-        home = os.environ.get("HOME")
-        if not home:
-            die("HOME not set")
-        return f"{home}/.claude/skills/{name}"
-
     def provision_skills(self, plan: BottlePlan, target: str) -> None:
         """Copy each of the agent's named skills from the host's
         ~/.claude/skills/<name>/ into the container's equivalent path.
@@ -345,7 +340,7 @@ class DockerBottleBackend(BottleBackend):
         )
 
         for n in agent.skills:
-            src = self._host_skill_dir(n)
+            src = host_skill_dir(n)
             if not os.path.isdir(src):
                 die(f"skill '{n}' disappeared from host between validation and copy at {src}.")
             dst = f"{skills_dir}/{n}"
@@ -423,14 +418,14 @@ class DockerBottleBackend(BottleBackend):
         keys_dir = "/root/.claude-bottle-keys"
 
         # ~/.ssh for node (700, owned by node).
-        self._docker_exec_root(container, ["mkdir", "-p", container_ssh])
-        self._docker_exec_root(container, ["chown", "node:node", container_ssh])
-        self._docker_exec_root(container, ["chmod", "700", container_ssh])
+        docker_mod.docker_exec_root(container, ["mkdir", "-p", container_ssh])
+        docker_mod.docker_exec_root(container, ["chown", "node:node", container_ssh])
+        docker_mod.docker_exec_root(container, ["chmod", "700", container_ssh])
 
         # /root/.claude-bottle-keys for root (700, root-owned).
-        self._docker_exec_root(container, ["mkdir", "-p", keys_dir])
-        self._docker_exec_root(container, ["chown", "root:root", keys_dir])
-        self._docker_exec_root(container, ["chmod", "700", keys_dir])
+        docker_mod.docker_exec_root(container, ["mkdir", "-p", keys_dir])
+        docker_mod.docker_exec_root(container, ["chown", "root:root", keys_dir])
+        docker_mod.docker_exec_root(container, ["chmod", "700", keys_dir])
 
         config_file = plan.stage_dir / "ssh_config"
         known_hosts_file = plan.stage_dir / "ssh_known_hosts"
@@ -459,8 +454,8 @@ class DockerBottleBackend(BottleBackend):
                 stdout=subprocess.DEVNULL,
                 check=True,
             )
-            self._docker_exec_root(container, ["chown", "root:root", container_key_path])
-            self._docker_exec_root(container, ["chmod", "600", container_key_path])
+            docker_mod.docker_exec_root(container, ["chown", "root:root", container_key_path])
+            docker_mod.docker_exec_root(container, ["chmod", "600", container_key_path])
 
             container_key_paths.append(container_key_path)
 
@@ -533,8 +528,8 @@ class DockerBottleBackend(BottleBackend):
             stdout=subprocess.DEVNULL,
             check=True,
         )
-        self._docker_exec_root(container, ["chown", "node:node", f"{container_ssh}/config"])
-        self._docker_exec_root(container, ["chmod", "600", f"{container_ssh}/config"])
+        docker_mod.docker_exec_root(container, ["chown", "node:node", f"{container_ssh}/config"])
+        docker_mod.docker_exec_root(container, ["chmod", "600", f"{container_ssh}/config"])
 
         if known_hosts_file.stat().st_size > 0:
             info(f"writing {container_ssh}/known_hosts")
@@ -543,15 +538,8 @@ class DockerBottleBackend(BottleBackend):
                 stdout=subprocess.DEVNULL,
                 check=True,
             )
-            self._docker_exec_root(container, ["chown", "node:node", f"{container_ssh}/known_hosts"])
-            self._docker_exec_root(container, ["chmod", "600", f"{container_ssh}/known_hosts"])
-
-    def _docker_exec_root(self, container: str, argv: list[str]) -> None:
-        subprocess.run(
-            ["docker", "exec", "-u", "0", container, *argv],
-            stdout=subprocess.DEVNULL,
-            check=True,
-        )
+            docker_mod.docker_exec_root(container, ["chown", "node:node", f"{container_ssh}/known_hosts"])
+            docker_mod.docker_exec_root(container, ["chmod", "600", f"{container_ssh}/known_hosts"])
 
     def provision_git(self, plan: BottlePlan, target: str) -> None:
         """If --cwd was set and the host cwd has a .git directory, copy

claude_bottle/backend/docker/util.py

@@ -48,6 +48,16 @@ def container_exists(name: str) -> bool:
     return bool(result.stdout.strip())
 
 
+def docker_exec_root(container: str, argv: list[str]) -> None:
+    """Run `docker exec -u 0` in the named container, check=True. Used
+    by SSH provisioning to chown/chmod files that need root."""
+    subprocess.run(
+        ["docker", "exec", "-u", "0", container, *argv],
+        stdout=subprocess.DEVNULL,
+        check=True,
+    )
+
+
 _SLUG_RE = re.compile(r"[^a-z0-9]+")
 
 

claude_bottle/backend/util.py

@@ -0,0 +1,18 @@
+"""Cross-backend utility helpers — host-side primitives shared by
+every backend implementation. Backend-specific helpers live one level
+deeper (e.g. claude_bottle/backend/docker/util.py)."""
+
+from __future__ import annotations
+
+import os
+
+from ..log import die
+
+
+def host_skill_dir(name: str) -> str:
+    """Return the host-side path for a named skill:
+    `$HOME/.claude/skills/<name>`. Dies if HOME is unset."""
+    home = os.environ.get("HOME")
+    if not home:
+        die("HOME not set")
+    return f"{home}/.claude/skills/{name}"