shall adopt should discover repos and PPAs

[SYKhayyat]

Problem

The repo: grammar exists and sync can add/remove package repositories, but there is no discovery. Users must manually write repo:apt:ppa:x/y or repo:dnf:https://... for repositories they already have configured.

Current behavior

Repos are declared, never detected. adopt discovers packages but not where they come from.

Proposed behavior

shall adopt (or shall adopt repos) should detect:

• apt: Parse /etc/apt/sources.list and /etc/apt/sources.list.d/*.list + sources.list.d/*.sources (DEB822 format)
• yum/dnf: Parse /etc/yum.repos.d/*.repo
• pacman: Parse /etc/pacman.conf (registered repos) and /etc/pacman.d/mirrorlist
• brew: brew tap
• snap: snap list (core/stable channels vs. edge/beta)
• flatpak: flatpak remotes
• nix: Channels and flake registries
• Windows: winget sources, scoop buckets

Each should produce a repo:BACKEND:SPEC declaration.

Scope questions

• Should PPAs that are just dead/404 be flagged rather than adopted?
• Official distro repos (e.g., archive.ubuntu.com) — skip or include?
• A user may have repos that conflict (e.g., two different Node version repos). Should adopt detect conflicts?