fix: resolve set() shadowing and sparse checkout ref issues

[Jah-yee]

Summary

This PR fixes two bugs reported in issue #179:

Bug 1: set() builtin shadowed by Click command

• File: src/apm_cli/cli.py
• Problem: set(_pre_download_results.keys()) invokes Click's config set command instead of Python's builtin set() because @config.command decorator defines def set(key, value) which overwrites the set = builtins.set alias.
• Fix: Use builtins.set(_pre_download_results.keys()) explicitly.

Bug 2: sparse checkout fetches wrong ref

• File: src/apm_cli/deps/github_downloader.py
• Problem: When no ref is provided, git fetch origin --depth=1 fetches all branch tips and FETCH_HEAD points to the wrong commit, causing the expected subdirectory to be missing after checkout.
• Fix: Always pass an explicit ref to git fetch - defaults to HEAD if not provided.

Changes

• cli.py: Changed set(...) to builtins.set(...) (line 1844)
• github_downloader.py: Changed conditional ref append to always pass ref (default to 'HEAD') (line 1076)

Testing

• Syntax validation passed (python3 -m py_compile)
• These fixes address the Release Validation CI job failure

Fixes #179

[Copilot]

Pull request overview

Fixes two reported regressions: set() being shadowed by a Click command in the CLI, and sparse checkout occasionally checking out the wrong commit by fetching an ambiguous ref.

Changes:

• Use builtins.set(...) explicitly to avoid calling the Click set command handler.
• Ensure sparse checkout always fetches an explicit ref (defaulting to HEAD) before checking out FETCH_HEAD.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/apm_cli/deps/github_downloader.py	Adjusts git fetch invocation to always include a ref for sparse checkout flows.
src/apm_cli/cli.py	Avoids set() shadowing by explicitly referencing builtins.set().