package: include zsh fork in Codex package

Summary:
- add a checked-in codex-zsh DotSlash manifest for supported Unix targets
- fetch the matching zsh fork artifact during package builds and install it at codex-resources/zsh/bin/zsh when available
- expose the bundled zsh fork through install-context and use it as the runtime zsh path for packaged installs
- remove the user/profile zsh_path config override now that the fork is package-managed
- use copyfile for package executable staging so local package smoke tests do not preserve platform-specific source metadata

Test Plan:
- just fmt
- just write-config-schema
- just fix -p codex-config -p codex-core -p codex-exec
- python3 -m py_compile scripts/codex_package/cli.py scripts/codex_package/layout.py scripts/codex_package/zsh.py
- python3 -m unittest discover scripts/codex_package
- cargo test -p codex-core default_zsh_path_sets_runtime_zsh_path
- cargo test -p codex-core session_new_fails_when_zsh_fork_enabled_without_packaged_zsh
- cargo test -p codex-core restricted_read_implicitly_allows_helper_executables
- cargo test -p codex-config

scripts/codex_package/zsh.py

@@ -0,0 +1,22 @@
+"""Fetch the patched zsh fork used by shell_zsh_fork."""
+
+from pathlib import Path
+
+from .dotslash import fetch_dotslash_executable
+from .targets import REPO_ROOT
+from .targets import TargetSpec
+
+
+ZSH_MANIFEST = REPO_ROOT / "scripts" / "codex_package" / "codex-zsh"
+ZSH_RESOURCE_PATH = Path("zsh") / "bin" / "zsh"
+
+
+def resolve_zsh_bin(spec: TargetSpec) -> Path | None:
+    return fetch_dotslash_executable(
+        spec,
+        manifest_path=ZSH_MANIFEST,
+        artifact_label="codex-zsh",
+        cache_key=f"{spec.target}-zsh",
+        dest_name="zsh",
+        missing_ok=True,
+    )