feat(opencode): configure formatters and provide them on PATH

Add formatter entries for nix, prettier (md/yaml/json/web), shell,
python, toml and lua, and install the matching tools via the opencode
home module so they are available wherever opencode runs.

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

config/opencode/config.json

@@ -1,6 +1,32 @@
 {
   "$schema": "https://opencode.ai/config.json",
   "lsp": true,
+  "formatter": {
+    "nixfmt": {
+      "command": ["nixfmt", "$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": {