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.

packages/tfcode/test/lib/filesystem.ts

@@ -0,0 +1,10 @@
+import path from "path"
+import { Effect, FileSystem } from "effect"
+
+export const writeFileStringScoped = Effect.fn("test.writeFileStringScoped")(function* (file: string, text: string) {
+  const fs = yield* FileSystem.FileSystem
+  yield* fs.makeDirectory(path.dirname(file), { recursive: true })
+  yield* fs.writeFileString(file, text)
+  yield* Effect.addFinalizer(() => fs.remove(file, { force: true }).pipe(Effect.orDie))
+  return file
+})