Improve smart threading for HTTPS URLs with pushurl

[jensens]

Summary

Improves smart threading performance by processing HTTPS URLs with pushurl in parallel instead of serially.

Problem

Currently, all HTTPS URLs are processed serially to avoid overlapping credential prompts. This is overly conservative for packages that define both:

• url = https://github.com/org/repo.git (public read)
• pushurl = git@github.com:org/repo.git (authenticated write)

These HTTPS URLs won't prompt for credentials (they're read-only/public), so they should be safe to process in parallel.

Solution

Updated _separate_https_packages() to check for pushurl presence:

• HTTPS without pushurl → serial processing (may prompt for credentials)
• HTTPS with pushurl → parallel processing (read-only, no prompts)
• SSH/fs/other → parallel processing (unchanged)

Benefits

✅ Performance: Common public repo pattern now processed in parallel
✅ Backward Compatible: No breaking changes, purely optimization
✅ Smart: Leverages semantic meaning of pushurl configuration

Changes

• src/mxdev/vcs/common.py: Updated _separate_https_packages() logic
• tests/test_git_additional.py: Added test_smart_threading_separates_https_with_pushurl()
• CHANGES.md: Documented performance improvement
• README.md: Updated Smart Threading documentation

Testing

• ✅ New unit test verifies separation logic
• ✅ All 185 tests pass (4 skipped)
• ✅ Pre-commit hooks pass

Example Configuration

[mypackage]
url = https://github.com/org/mypackage.git    # Public read - now parallel!
pushurl = git@github.com:org/mypackage.git    # Authenticated write