Move shared git package from operator to pkg/git

The operator's git package (CloneConfig, Client interface, LimitedFs,
DefaultGitClient) is needed by both the operator and the CLI for
git-based skill installation. Move it to pkg/git/ so both can share it.

Changes during move:
- Replace controller-runtime/pkg/log with log/slog
- Add WithAuth ClientOption for authenticated clones
- Remove runtime.GC() call from Cleanup (unnecessary; Go GC handles it)
- Update tests to use t.Context() instead of controller-runtime logger
- Rewrite tests with testify assertions and t.TempDir()
- Drop trivial field-accessibility tests and slow network-dependent tests

No functional changes to the operator — the package was not yet imported
by any operator code.

Relates to #4015

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JAORMX