feat: --version flag and upgrade check

[Champion2005]

Summary

Add --version flag and automatic upgrade-available warning on all CLI invocations.

Changes

• src/netbox_data_puller/version_check.py — New module: PyPI version check with 7-day cache, silent failure, NBPULL_NO_UPDATE_CHECK opt-out
• src/netbox_data_puller/cli.py — @app.callback with --version eager flag + atexit-based upgrade warning
• pyproject.toml — Added packaging>=24.0 as explicit dependency
• tests/test_version_check.py — 9 unit tests for version check module
• tests/test_cli.py — 4 new tests for --version flag and upgrade registration
• CHANGELOG.md + docs/commands.md — Documentation updates

Requirements

• R1: --version flag ✅
• R2: Upgrade warning on all invocations ✅
• R3: 7-day cached PyPI check ✅
• R4: Silent failure ✅
• R5: Rich console to stderr ✅
• R6: NBPULL_NO_UPDATE_CHECK opt-out ✅
• R7: mypy --strict + ruff ✅
• R8: Full test coverage (13 new tests) ✅