fix: align Python version floor with KiCad's macOS bundle (3.9)

KiCad ships Python 3.9 on macOS (both v9 and v10), and `pcbnew.so` is
compiled against that interpreter. The previous `>=3.10` floor was
unreachable on macOS and docs referenced nonexistent
`python3.11/site-packages` paths. Lower the floor and correct the docs;
no source changes needed — all 49 files in `python/` already compile
clean on 3.9.

Also add `.venv/` to `.gitignore` (dotted form used by `python -m venv`).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/macos-config.example.json

@@ -5,7 +5,7 @@
       "args": ["/Users/YOUR_USERNAME/MCP/KiCAD-MCP-Server/dist/index.js"],
       "env": {
         "NODE_ENV": "production",
-        "PYTHONPATH": "/Applications/KiCad/KiCad.app/Contents/Frameworks/Python.framework/Versions/Current/lib/python3.11/site-packages",
+        "PYTHONPATH": "/Applications/KiCad/KiCad.app/Contents/Frameworks/Python.framework/Versions/Current/lib/python3.9/site-packages",
         "LOG_LEVEL": "info",
         "KICAD_AUTO_LAUNCH": "false"
       },