fix: correct mdformat regex to properly exclude virtual environments

The previous regex pattern was not correctly excluding files at the root level
of excluded directories like .venv/. The negative lookahead required the pattern
to match after a slash, but root-level directories don't have a preceding slash.

Changed from:
  (?!.*/(\.venv|...))
To:
  (?!.*(^|/)(\.venv|...))

This ensures that files like .venv/lib/python3.13/site-packages/test.md are
properly excluded, preventing CI failures when mdformat tries to process
third-party library files that don't conform to our formatting standards.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: SimplicityGuy

.pre-commit-config.yaml

@@ -35,7 +35,7 @@ repos:
           - mdformat-black==0.1.1
           - mdformat-gfm==0.4.1
           - mdformat-tables==1.0.0
-        files: '^(?!.*/(\.venv|venv|__pycache__|\.git|node_modules|\.egg-info|\.pytest_cache|\.mypy_cache|\.ruff_cache|dist|build)/).*\.md$'
+        files: '^(?!.*(^|/)(\.venv|venv|__pycache__|\.git|node_modules|\.egg-info|\.pytest_cache|\.mypy_cache|\.ruff_cache|dist|build)/).*\.md$'
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
     rev: db90487f48a9dd992d243ef63c156eaffddeaf28  # frozen: v0.12.11