refactor: migrate from Nx to Turborepo

Removed Nx in favor of Turborepo for simpler, more focused task orchestration.

Why Turborepo:
- Simpler and more focused than Nx
- Free Vercel Remote Cache for everyone
- Better alignment with cigen's philosophy of using platform features
- Native support for self-hosted caching (S3, Redis)
- Less overhead, cleaner configuration

Changes:
- Removed nx.json and project.json
- Added turbo.json with task definitions for build, test, lint, fmt, clippy
- Turborepo will be used for task dependency tracking and caching

Next steps:
- Update cigen to read turbo.json for task graph
- Create .cigen/ config for self-hosting
- Generate dual-provider CI (CircleCI + GitHub Actions)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ndbroadbent

nx.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "https://turbo.build/schema.json",
+  "tasks": {
+    "build": {
+      "dependsOn": ["^build"],
+      "outputs": ["target/**"]
+    },
+    "test": {
+      "dependsOn": ["build"],
+      "outputs": []
+    },
+    "lint": {
+      "outputs": []
+    },
+    "fmt": {
+      "outputs": []
+    },
+    "clippy": {
+      "outputs": []
+    }
+  }
+}