Add minimal control plane with REST API, database, and reconciler

- REST API (chi router): sandbox CRUD, exec, pause/resume, file write/read
- PostgreSQL persistence via pgx/v5 + sqlc (sandboxes table with goose migration)
- Connect RPC client to host agent for all VM operations
- Reconciler syncs host agent state with DB every 30s (detects TTL-reaped sandboxes)
- OpenAPI 3.1 spec served at /openapi.yaml, Swagger UI at /docs
- Added WriteFile/ReadFile RPCs to hostagent proto and implementations
- File upload via multipart form, download via JSON body POST
- sandbox_id propagated from control plane to host agent on create

sqlc.yaml

@@ -0,0 +1,11 @@
+version: "2"
+sql:
+  - engine: "postgresql"
+    queries: "db/queries"
+    schema: "db/migrations"
+    gen:
+      go:
+        package: "db"
+        out: "internal/db"
+        sql_package: "pgx/v5"
+        emit_json_tags: true