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>

CONTRIBUTING.md

@@ -21,7 +21,7 @@ Thank you for your interest in contributing to the KiCAD MCP Server! This guide
 
 - **KiCAD 9.0 or higher** - [Download here](https://www.kicad.org/download/)
 - **Node.js v18+** - [Download here](https://nodejs.org/)
-- **Python 3.10+** - Should come with KiCAD, or install separately
+- **Python 3.9+** - Comes bundled with KiCAD (macOS builds ship Python 3.9; Linux/Windows builds ship Python 3.11)
 - **Git** - For version control
 
 ### Platform-Specific Setup