chore: remove opencode version override once nixpkgs updates to 1.3.10+

[stackptr]

Context

PR #423 adds a local override for opencode 1.3.10 because the flake lock is temporarily stuck and can't be updated to pick up the fix for the web UI crash (TypeError: e.diffs.map is not a function).

When to close

Once nix flake update succeeds and the nixpkgs pin advances past opencode 1.3.10, revert #423:

• Remove overlays/opencode.nix
• Remove packages/opencode/package.nix
• Remove the overlay entry from overlays/default.nix

Check

nix eval .#nixosConfigurations.glyph.pkgs.opencode.version

Should return 1.3.10 or higher from nixpkgs directly once the override is removed.