opencode: add agent skills support

Adds support for OpenCode Agent Skills by managing skill definitions
under "/opencode/skill/<name>/SKILL.md" via `programs.opencode.skills`.

Documentation: https://opencode.ai/docs/skills/

tests/modules/programs/opencode/skills-inline.nix

@@ -0,0 +1,25 @@
+{
+  programs.opencode = {
+    enable = true;
+    skills = {
+      git-release = ''
+        ---
+        name: git-release
+        description: Create consistent releases and changelogs
+        ---
+
+        ## What I do
+
+        - Draft release notes from merged PRs
+        - Propose a version bump
+        - Provide a copy-pasteable `gh release create` command
+      '';
+    };
+  };
+
+  nmt.script = ''
+    assertFileExists home-files/.config/opencode/skill/git-release/SKILL.md
+    assertFileContent home-files/.config/opencode/skill/git-release/SKILL.md \
+      ${./git-release-SKILL.md}
+  '';
+}