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/env/index.ts

@@ -0,0 +1,28 @@
+import { Instance } from "../project/instance"
+
+export namespace Env {
+  const state = Instance.state(() => {
+    // Create a shallow copy to isolate environment per instance
+    // Prevents parallel tests from interfering with each other's env vars
+    return { ...process.env } as Record<string, string | undefined>
+  })
+
+  export function get(key: string) {
+    const env = state()
+    return env[key]
+  }
+
+  export function all() {
+    return state()
+  }
+
+  export function set(key: string, value: string) {
+    const env = state()
+    env[key] = value
+  }
+
+  export function remove(key: string) {
+    const env = state()
+    delete env[key]
+  }
+}