flake: add coding-agent validation workflow

opencode.json

@@ -0,0 +1,23 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "mcp": {
+    "nixos": {
+      "type": "local",
+      "command": ["mcp-nixos"],
+      "enabled": true,
+      "timeout": 30000
+    },
+    "github": {
+      "type": "remote",
+      "url": "https://api.githubcopilot.com/mcp/",
+      "enabled": true,
+      "oauth": false,
+      "headers": {
+        "Authorization": "Bearer {env:GITHUB_PERSONAL_ACCESS_TOKEN}",
+        "X-MCP-Readonly": "true",
+        "X-MCP-Toolsets": "repos,pull_requests,actions"
+      },
+      "timeout": 30000
+    }
+  }
+}