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/manifest.py

@@ -119,9 +119,9 @@ class Bottle:
         if "runtime" in d:
             die(
                 f"bottle '{name}' has a 'runtime' field, which is no longer "
-                f"supported. gVisor (runsc) is now auto-detected when "
-                f"registered with Docker; remove the 'runtime' field from "
-                f"the bottle definition."
+                f"supported. gVisor (runsc) is now auto-detected by the "
+                f"backend; remove the 'runtime' field from the bottle "
+                f"definition."
             )
 
         env: dict[str, str] = {}