Add explicit Fedora Atomic install methods and harden release installs

[archham]

Summary

This PR updates the Fedora Atomic / rpm-ostree install flow so users can choose between two supported Topgrade ownership models:

1. COPR / rpm-ostree

   • installs Topgrade as a layered RPM
   • updates Topgrade through rpm-ostree
   • disables Topgrade self-update
   • restricts the added lilay/topgrade COPR repository with includepkgs=topgrade

2. Official upstream binary

   • installs Topgrade to ~/.local/bin/topgrade
   • does not add a COPR repository
   • does not layer a package into the OS
   • keeps Topgrade self-update enabled
   • verifies the GitHub release asset before activation

The goal is to avoid presenting either path as universally “right” or “wrong”. Instead, the installer documents and supports both trust/ownership models clearly.

What changed

• Added Atomic install method selection:

  • --install-method=binary
  • --install-method=copr

• Added aliases:

  • --binary
  • --copr

• Interactive Fedora Atomic sessions now prompt when no method is specified.

• Non-interactive Fedora Atomic sessions default to the binary method.

• COPR / rpm-ostree remains supported for users who want package-manager integration.

• The COPR repo file is restricted with:

  includepkgs=topgrade

• The upstream binary install path now:

  • queries GitHub release metadata with explicit headers/status handling
  • selects the expected Linux release asset
  • verifies the GitHub-provided SHA-256 digest when present
  • checks archive paths before extraction
  • smoke-tests topgrade --version
  • activates the new binary atomically

• Existing config handling performs small policy repairs with backups:

  • COPR/package-managed installs set no_self_update = true
  • user-local binary installs keep self-update enabled
  • Atomic installs keep rpm_ostree = true

• Updated English and German README documentation with a comparison of the two Fedora Atomic methods.

Why

The COPR security/trust scope is narrower than a blanket “trust all of COPR”: enabling lilay/topgrade trusts that specific COPR project, not unrelated COPR repositories.

However, the upstream binary path is still useful for users who prefer not to add a persistent third-party RPM repository or layer Topgrade into an Atomic host.

This PR makes that tradeoff explicit:

• COPR / rpm-ostree = package-manager integration, root/reboot required, trust in the specific COPR project
• upstream binary = user-local install, no host package layer, trust in upstream GitHub releases

Validation

• bash -n install-topgrade.sh
• ShellCheck with --severity=error
• Simulated Fedora Atomic binary install pinned to v17.9.0
  • release asset selected correctly
  • SHA-256 digest verified
  • binary installed to ~/.local/bin/topgrade
  • topgrade --version returned the expected version
  • config contained rpm_ostree = true
  • self-update remained enabled
• Simulated Fedora Atomic COPR install
  • COPR repo file installed
  • repo file contained includepkgs=topgrade
  • config contained rpm_ostree = true
  • config contained no_self_update = true
  • reboot message was shown
• Simulated non-interactive Fedora Atomic install with no method specified
  • defaulted to the binary method
• git diff --check
• GitHub ShellCheck passed for push and pull request runs

[Pat9496]

Nice work!