refactor(docker): inline skills_copy_into into provision_skills

The copy logic was Docker-specific (docker exec mkdir / rm -rf,
docker cp); it had no reason to live in a top-level skills module.
Pull the body into DockerBottleBackend.provision_skills.

skills.py keeps the host-side discovery + validation
(host_skill_dir, host_skill_exists, require_host_skill,
skills_validate_all). The orphaned CONTAINER_HOME /
CONTAINER_SKILLS_DIR constants and the now-unused subprocess + info
imports are removed.

claude_bottle/skills.py

@@ -1,17 +1,11 @@
-"""Skill copier: host's ~/.claude/skills/<name>/ -> container's
-~/.claude/skills/<name>/, preserving directory structure."""
+"""Skill discovery and host-side validation. The copy step itself
+lives on the backend (e.g. DockerBottleBackend.provision_skills)."""
 
 from __future__ import annotations
 
 import os
-import subprocess
 
-from .log import die, info
-
-CONTAINER_HOME = os.environ.get("CLAUDE_BOTTLE_CONTAINER_HOME", "/home/node")
-CONTAINER_SKILLS_DIR = os.environ.get(
-    "CLAUDE_BOTTLE_CONTAINER_SKILLS_DIR", f"{CONTAINER_HOME}/.claude/skills"
-)
+from .log import die
 
 
 def host_skill_dir(name: str) -> str:
@@ -38,39 +32,3 @@ def skills_validate_all(names: list[str]) -> None:
     that's already known to fail."""
     for n in names:
         require_host_skill(n)
-
-
-def skills_copy_into(container: str, names: list[str]) -> None:
-    """For each named skill, ensure the parent dir exists, wipe any
-    prior copy, then `docker cp <host>/. <container>:<dst>/` so the
-    contents are copied into a freshly-created destination dir."""
-    if not names:
-        return
-
-    subprocess.run(
-        ["docker", "exec", container, "mkdir", "-p", CONTAINER_SKILLS_DIR],
-        stdout=subprocess.DEVNULL,
-        check=True,
-    )
-
-    for n in names:
-        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"{CONTAINER_SKILLS_DIR}/{n}"
-        info(f"copying skill {n} into {container}:{dst}")
-        subprocess.run(
-            ["docker", "exec", container, "rm", "-rf", dst],
-            stdout=subprocess.DEVNULL,
-            check=True,
-        )
-        subprocess.run(
-            ["docker", "exec", container, "mkdir", "-p", dst],
-            stdout=subprocess.DEVNULL,
-            check=True,
-        )
-        subprocess.run(
-            ["docker", "cp", f"{src}/.", f"{container}:{dst}/"],
-            stdout=subprocess.DEVNULL,
-            check=True,
-        )