refactor(env): make env resolution backend-agnostic

resolve_env_into(...) becomes resolve_env(manifest, agent) -> ResolvedEnv
(forwarded names + literals). The docker backend now owns env-file /
argv serialization and the --env-file newline check. Also drops stray
Docker references from manifest.py, pipelock.py, util.py, and trims
the duplicated command list from cli.py's docstring (usage() in
claude_bottle/cli/__init__.py is now the only listing).

cli.py

@@ -1,19 +1,6 @@
 #!/usr/bin/env python3
-"""cli.py — manage claude-bottle containers.
-
-usage: cli.py <command> [args...]
-
-Commands:
-  build     build (or rebuild) the claude-bottle Docker image.
-  cleanup   stop and remove all active claude-bottle containers.
-  edit      open an agent in vim for editing.
-  info      print env, skills, and prompt details for a named agent.
-  init      interactively create a new agent and add it to claude-bottle.json.
-  list      list available agents or active containers.
-  start     boot a sandboxed container for a named agent and attach an
-            interactive claude-code session. The container is torn down
-            when the session ends.
-"""
+"""cli.py — entry point for the claude-bottle CLI. Run with --help (or
+no args) for the command list."""
 
 from __future__ import annotations