Add system and Nginx doctor diagnostics

[franckferman]

Summary

• add read-only doctor commands for system, disk, service, network, and Nginx diagnosis
• correlate Nginx syntax, active configuration metadata, service state, listeners, paths, security frameworks, filesystems, logs, and optional HTTP HEAD probes
• keep logs opt-in, bounded, and redacted by default
• document operational use and add an installed-package CI smoke test

Validation

• 94 unit and integration tests on local Python 3.14
• 94 tests in a clean Python 3.9 container
• mypy clean
• real Nginx 1.26.3 healthy and broken-configuration integration scenarios
• wheel build, install, version, and doctor smoke test