chore: exclude .python-version from package distribution

Add .python-version to the sdist exclusion list. This file is used by
tools like pyenv for local development but should not be distributed
to end users.

Analysis of PyPI 0.1.1 package showed .python-version was included in
the source distribution. This change removes it from future releases.

Package contents verification:
- ✅ .python-version successfully excluded
- ✅ Only essential files remain (src/, LICENSE, README, pyproject.toml, .gitignore)
- ✅ Wheel distribution unchanged (already optimal)

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: mldangelo

pyproject.toml

@@ -49,6 +49,7 @@ Documentation = "https://www.promptfoo.dev/docs"
 exclude = [
     "/.github",
     "/.gitignore",
+    "/.python-version",
     "/.release-please-manifest.json",
     "/release-please-config.json",
     "/renovate.json",