Remove unused opencode.json configuration.

The file is not part of the BilHej toolchain and is no longer needed in
the repository root.

opencode.json

@@ -1,21 +0,0 @@
-{
-  "$schema": "https://opencode.ai/config.json",
-  "instructions": ["CODING_GUIDELINES.md", "REQUIREMENTS.md"],
-  "lsp": true,
-  "formatter": true,
-  "permission": {
-    "edit": "ask",
-    "bash": "ask"
-  },
-  "tools": {
-    "websearch": true,
-    "codesearch": true
-  },
-  "compaction": {
-    "auto": true,
-    "prune": true
-  },
-  "watcher": {
-    "ignore": ["node_modules/**", ".git/**", "dist/**", "build/**", "**/target/**"]
-  }
-}