fix(observer-install): skip pipx pkg check when pipx is on PATH
Linux dry-run preflight reported pipx as both ✓ tool and ✗ system-package when pipx was installed outside the distro's package manager (e.g. via uv or pip). Tool detection is the canonical source of truth — short-circuit the rpm/dpkg check for pipx-equivalent package names when 'command -v pipx' succeeds.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>