v1.20.0

Adopt the cobra CLI framework

netwp's CLI is now built on cobra, replacing the hand-rolled os.Args dispatch. Every existing command and flag keeps working exactly as before — scripts and docs stay valid — but you now get:

• Per-command help: netwp scan --help, netwp alias set --help, etc.
• Native shell completion: netwp completion <bash|zsh|fish|powershell>.
• Structured grouped commands: alias, class, watch, and iface are proper parent + subcommands.

Errors still print in netwp's netwp: <err> style. This is a cmd/netwp-only change; the core/adapter/tui layers are untouched. Closes #78.

Also in this release: GitHub issue templates and a pull request template (community-standards checklist).