refactor: apply minimal tfcode branding

- Rename packages/opencode → packages/tfcode (directory only)
- Rename bin/opencode → bin/tfcode (CLI binary)
- Rename .opencode → .tfcode (config directory)
- Update package.json name and bin field
- Update config directory path references (.tfcode)
- Keep internal code references as 'opencode' for easy upstream sync
- Keep @opencode-ai/* workspace package names

This minimal branding approach allows clean merges from upstream
opencode repository while providing tfcode branding for users.

.tfcode/opencode.jsonc

@@ -0,0 +1,18 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "provider": {
+    "opencode": {
+      "options": {},
+    },
+  },
+  "permission": {
+    "edit": {
+      "packages/opencode/migration/*": "deny",
+    },
+  },
+  "mcp": {},
+  "tools": {
+    "github-triage": false,
+    "github-pr-search": false,
+  },
+}