feat(opencode): install kepano/obsidian-skills into ~/.agents/skills

Add obsidian-skills as a flake input (flake = false) and map each
skill subdirectory into ~/.agents/skills/<skill>, alongside the
existing local skills. Updates flow through `nix flake update
obsidian-skills`.

flake.lock

@@ -672,6 +672,22 @@
         "type": "github"
       }
     },
+    "obsidian-skills": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1778185448,
+        "narHash": "sha256-2kr8fmkuxRLaTy2O+dTv9K+75mRIqkLmaArw2/hj5uc=",
+        "owner": "kepano",
+        "repo": "obsidian-skills",
+        "rev": "ac9398734fe719565809f7a6048b05c36b1ca38f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "kepano",
+        "repo": "obsidian-skills",
+        "type": "github"
+      }
+    },
     "pre-commit-hooks-nix": {
       "inputs": {
         "flake-compat": [
@@ -714,6 +730,7 @@
         "nix-homebrew": "nix-homebrew",
         "nixos-hardware": "nixos-hardware",
         "nixpkgs": "nixpkgs_2",
+        "obsidian-skills": "obsidian-skills",
         "rust-overlay": "rust-overlay_2",
         "simple-nixos-mailserver": "simple-nixos-mailserver",
         "snowfall-lib": "snowfall-lib",