fix: handle read-only .git objects on Windows in shutil.rmtree calls

[ryanfk]

Summary

• On Windows, Git marks pack files and loose objects as read-only. When APM tries to remove a previously-cloned apm_modules directory or strip the .git folder after cloning, shutil.rmtree raises [WinError 5] Access is denied.
• Adds a _remove_readonly error handler that clears the read-only flag before retrying the delete.
• Wires the handler into all five shutil.rmtree calls in github_downloader.py, replacing both bare calls and ignore_errors=True (which silently left files behind on Windows).

Reproduction

apm install dev.azure.com/myorg/myproject/myrepo
# First install fails for any reason (e.g. wrong branch)
# Second install attempt hits:
# ❌ Failed to install myorg/myproject/myrepo: [WinError 5] Access is denied:
#    'C:\...\apm_modules\myorg\myproject\myrepo\.git\objects\02\...'

Fix

def _remove_readonly(func, path, _excinfo):
    """Clear read-only flag on Windows before retrying delete."""
    os.chmod(path, stat.S_IWRITE)
    func(path)

# Before:
shutil.rmtree(target_path)
# After:
shutil.rmtree(target_path, onerror=_remove_readonly)

Test plan

• Verified on Windows 11 — apm install now succeeds after a previously failed install leaves read-only .git objects in apm_modules/
• No-op on Linux/macOS — the onerror handler only triggers when the default delete fails, which doesn't happen on systems without read-only git objects

Fixes #90

🤖 Generated with Claude Code