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/src/util/signal.ts

@@ -0,0 +1,12 @@
+export function signal() {
+  let resolve: any
+  const promise = new Promise((r) => (resolve = r))
+  return {
+    trigger() {
+      return resolve()
+    },
+    wait() {
+      return promise
+    },
+  }
+}