Add Opencode and Ollama modules; update configs

Enable Opencode and Ollama in home modules and import their Nix files.
Disable the deprecated system‑module opencode entries and remove it from
the
core packages list.
Add a new GitHub CLI module and a convenient `upf-gh` alias for flake
updates using a GitHub token.
Update the `stylix` configuration to include Opencode styling.
Refresh several flake.lock entries with newer revisions and hashes.

hosts/default/system-modules.nix

@@ -68,7 +68,6 @@ _: {
     */
     exercism.enable = true; # Exercism: coding practice platform
     lazygit.enable = false; # Lazygit: simple TUI for Git
-    opencode.enable = false; # OpenCode: tools for coding and development
     jan.enable = true; # Jan: AI chat UI
     logisim-evolution.enable = false; # Logisim-Evolution: Digital logic designer and simulator