Add global --system-certs flag and apply it to all HTTPS requests

[Copilot]

This change introduces a global --system-certs CLI flag so users can force HTTPS calls to use the OS certificate store. It wires this behavior through shared settings and applies it consistently across GitHub API resolution and archive download paths.

• CLI surface

  • Adds a new global option:
    • --system-certs
  • Captured in the Typer callback and stored in runtime settings.

• TLS verification behavior

  • Extends Settings with:
    • use_system_certs: bool
    • tls_verify property (returns system CA file/path when enabled; defaults to standard verification otherwise).
  • Uses settings.tls_verify in all outbound requests.get(...) call sites:
    • latest release lookup
    • commit SHA lookup
    • release zip download

• Test coverage updates

  • Adds focused assertions for:
    • flag-driven setting mutation (--system-certs)
    • verify= propagation in GitHub/download request calls
    • system-cert path usage when enabled
  • Updates shared test fixtures to reset settings.use_system_certs and avoid state bleed between tests.

• Docs

  • Updates command docs and README usage examples to include --system-certs.

skillup --system-certs add myorg/skills

[aersam]

Does this work for windows cert store? Would not using truststore be better?

@copilot