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.

crates/herald-server/src/sandbox/mod.rs

@@ -25,8 +25,6 @@ const DEVCONTAINER_PATHS: [&str; 2] = [".devcontainer/devcontainer.json", ".devc
 /// Sandbox-related runtime configuration.
 #[derive(Clone)]
 pub struct SandboxConfig {
-    /// Whether the bot should run its tools inside a sandbox container.
-    pub enabled: bool,
     /// Container runtime binary to drive (e.g. `docker`, `podman`).
     pub runtime: ContainerRuntime,
     /// Maximum number of tool-calling iterations per agent run.