[fix] detect remote default branch instead of hardcoding main (#573)

[edenfunf]

What

apm install failed when a dependency repo uses any default branch other
than main (e.g. master, develop). Before this fix, a clone with
--branch=main was always attempted, producing:

fatal: Remote branch main not found in upstream origin

After this fix, when no #ref is given in apm.yml, git clones without
--branch (letting the remote HEAD decide), and the actual default branch
name is read from repo.active_branch.name for subsequent operations.

Why

resolve_git_reference defaulted to "main" when dep_ref.reference was
empty and passed it as --branch=main to every shallow clone, breaking any
repo whose default branch is not main. This is not intended behaviour --
the dependency declaration carries no branch, so APM should follow whatever
branch the remote advertises as HEAD.

Refs #573.

How

In resolve_git_reference (github_downloader.py):

• ref = dep_ref.reference or "main" changed to ref = dep_ref.reference or None
• Shallow clone: build clone_kwargs with branch only when ref is set,
  matching the existing pattern in download_subdirectory_package
• After a no-ref clone, populate ref_name from repo.active_branch.name
  instead of the hardcoded string
• In the except GitCommandError fallback, raise immediately when ref is
  None -- the failure is not a branch-not-found issue and the ref-lookup
  loop that follows would be nonsensical without a target ref
• Artifactory fast-path (no remote to query) keeps "main" as its fallback
  via a local effective_ref = ref or "main"

Test

• Added test_resolve_git_reference_no_ref_uses_remote_head: mocks Repo.clone_from
  with active_branch.name = 'master', asserts the resolved ref_name is
  'master' and that clone_from was called without a branch kwarg
• Existing test_resolve_git_reference_branch (explicit #main) continues to pass
• Full unit suite: python -m pytest tests/unit/ -q --ignore=tests/unit/test_audit_report.py
  -- 3527 passed
• The one pre-existing failure (test_credential_fill_used_when_no_env_token) is
  unrelated to this change and fails on the unmodified main branch too

[Copilot]

Pull request overview

Fixes apm install for Git dependencies that do not use main as the remote default branch by avoiding --branch=main when no #ref is specified, and by reading the checked-out default branch name after cloning.

Changes:

• Update resolve_git_reference() to treat a missing #ref as "use remote HEAD" (omit --branch) and to record the detected default branch name from the clone.
• Preserve the existing Artifactory behavior by continuing to fall back to "main" when no remote can be queried.
• Add a unit test asserting that no-ref dependencies clone without a branch kwarg and return the detected default branch name.

Show a summary per file

File	Description
src/apm_cli/deps/github_downloader.py	Stop defaulting missing refs to main; omit branch for shallow clones when no ref is provided; capture the remote default branch name post-clone.
tests/test_github_downloader.py	Add regression test ensuring no-ref cloning does not pass branch and resolves ref_name from the repo's active branch.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2