Replace openrouter-rs with in-tree client and require sandbox

Remove the openrouter-rs dependency in favor of a minimal in-tree
OpenRouter chat-completions client, and drop the BOT_NAME and
SANDBOX_ENABLED config options. Reviews now always run inside the
sandbox, and the review prompt asks the model to read files with the
available tools instead of embedding the diff.

.env.example

@@ -1,5 +1,4 @@
 HTTP_PORT=3000
-BOT_NAME=Herald
 
 WEBHOOK_SIG_HEADER_SECRET=
 
@@ -20,7 +19,6 @@ RUST_BACKTRACE=1
 
 METRICS_BIND_ADDR=
 
-# Sandboxed tool execution (optional)
-SANDBOX_ENABLED=false
+# Sandboxed tool execution
 CONTAINER_RUNTIME=docker
-SANDBOX_MAX_ITERATIONS=8
+SANDBOX_MAX_ITERATIONS=8