refactor(opencode): generate config.json from the home module

Build opencode's config.json with pkgs.formats.json instead of shipping
a static file, pinning each formatter command to its store-path binary
via lib.getExe. Drops the standalone config/opencode/config.json.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config/opencode/config.json

@@ -1,43 +0,0 @@
-{
-  "$schema": "https://opencode.ai/config.json",
-  "lsp": true,
-  "formatter": {
-    "nixfmt": {
-      "command": ["nixfmt", "$FILE"],
-      "extensions": [".nix"]
-    },
-    "prettier": {
-      "command": ["prettier", "--write", "$FILE"],
-      "extensions": [".md", ".yaml", ".yml", ".json", ".css", ".html", ".js", ".ts", ".jsx", ".tsx"]
-    },
-    "shfmt": {
-      "command": ["shfmt", "-w", "$FILE"],
-      "extensions": [".sh", ".bash"]
-    },
-    "ruff": {
-      "command": ["ruff", "format", "$FILE"],
-      "extensions": [".py"]
-    },
-    "taplo": {
-      "command": ["taplo", "format", "$FILE"],
-      "extensions": [".toml"]
-    },
-    "stylua": {
-      "command": ["stylua", "$FILE"],
-      "extensions": [".lua"]
-    }
-  },
-  "disabled_providers": ["opencode"],
-  "provider": {
-    "halo": {
-      "npm": "@ai-sdk/openai-compatible",
-      "name": "Halo",
-      "options": {
-        "baseURL": "http://halo:8000/v1"
-      },
-      "models": {
-        "coder": { "name": "coder" }
-      }
-    }
-  }
-}