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).

claude_bottle/pipelock.py

@@ -107,7 +107,7 @@ class PipelockProxyPlan:
 class PipelockProxy(ABC):
     """The pipelock egress proxy. Encapsulates the YAML-config
     generation; the sidecar's start/stop lifecycle is backend-specific
-    and lives on concrete subclasses (e.g. DockerPipelockProxy)."""
+    and lives on concrete subclasses."""
 
     def prepare(
         self, bottle: Bottle, slug: str, yaml_path: Path