Suppress dead_code and unused_imports warnings in prompt_builder module

Module is scaffolded but not yet wired into callers — allow dead code
at the module level until integration is complete.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/ai/mod.rs

@@ -17,6 +17,7 @@ pub(crate) mod attachment_utils;
 pub mod aws_credentials;
 #[cfg(not(target_family = "wasm"))]
 pub mod bedrock;
+#[allow(dead_code)]
 pub mod prompt_builder;
 pub(crate) mod block_context;
 pub(crate) mod blocklist;