feat: replace local opencode with upstream flake input v1.1.27

- Add `opencode` flake input pinned to v1.1.27 for stability
- Remove local `pkgs/opencode` derivation and overlay
- Add `opencode` and `opencode-desktop` to `pkgs/default.nix` from input

flake.lock

@@ -16,9 +16,45 @@
         "type": "github"
       }
     },
+    "nixpkgs_2": {
+      "locked": {
+        "lastModified": 1768569498,
+        "narHash": "sha256-bB6Nt99Cj8Nu5nIUq0GLmpiErIT5KFshMQJGMZwgqUo=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "be5afa0fcb31f0a96bf9ecba05a516c66fcd8114",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "opencode": {
+      "inputs": {
+        "nixpkgs": "nixpkgs_2"
+      },
+      "locked": {
+        "lastModified": 1768911228,
+        "narHash": "sha256-iJ4WLNyP9i8baxp1FJJLv2VBD3XcE8nCruwO3GmSmK4=",
+        "owner": "anomalyco",
+        "repo": "opencode",
+        "rev": "e521fee0023a604bb6d5ef39b4b892cbf1a0f9d4",
+        "type": "github"
+      },
+      "original": {
+        "owner": "anomalyco",
+        "ref": "v1.1.27",
+        "repo": "opencode",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "opencode": "opencode"
       }
     }
   },