pytest plugin: Improve performance via caching (#472)

# Problem

Git, Mercurial, and Subversion repositories are unnecessarily reinitialized for each test.

- We're not utilizing session-based scoping.
  - A single initial repo could be created, then copied to [`tmp_path`](https://docs.pytest.org/en/8.3.x/how-to/tmp_path.html#the-tmp-path-fixture) using [`shutil.copytree`](https://docs.python.org/3/library/shutil.html#shutil.copytree) ([source](https://github.com/python/cpython/blob/v3.13.0/Lib/shutil.py#L550-L605)).

Issue #471 highlighted this inefficiency, where benchmarks showed tens of thousands of redundant functional calls.

# Improvement

```
❯ hyperfine -L branch master,pytest-plugin-fixture-caching 'git checkout {branch} && py.test'
Benchmark 1: git checkout master && py.test
  Time (mean ± σ):     32.062 s ±  0.869 s    [User: 41.391 s, System: 9.931 s]
  Range (min … max):   30.878 s … 33.583 s    10 runs

Benchmark 2: git checkout pytest-plugin-fixture-caching && py.test
  Time (mean ± σ):     14.659 s ±  0.495 s    [User: 16.351 s, System: 4.433 s]
  Range (min … max):   13.990 s … 15.423 s    10 runs

Summary
  git checkout pytest-plugin-fixture-caching && py.test ran
    2.19 ± 0.09 times faster than git checkout master && py.test
```

# Changes

## Pytest fixtures overhaul

1. Create a base VCS repo.
2. For subsequent tests, copy and modify from this template.

Author: tony

CHANGES

@@ -17,6 +17,11 @@ $ pip install --user --upgrade --pre libvcs
 
 ### Breaking changes
 
+#### pytest plugin: Improve performacne via session-based scoping (#472)
+
+Improved test execution speed by over 54% by eliminated repetitive repository reinitialization between test runs.
+Git, Subversion, and Mercurial repositories are now cached from an initial starter repository
+
 #### pytest fixtures: `git_local_clone` renamed to `example_git_repo` (#468)
 
 Renamed `git_local_clone` to `example_git_repo` for better understandability in