feat: add Copilot optimizer to reduce premium interaction consumption

Implement request classification, tool result merging, compact detection,
deterministic request IDs, and warmup downgrade for Copilot proxy.

The root cause was x-initiator being hardcoded to "user", making Copilot
count every API request (including tool callbacks and agent continuations)
as a separate premium interaction. The optimizer dynamically classifies
requests as "user" or "agent" based on message content analysis.

Closes #1813

src-tauri/src/proxy/mod.rs

@@ -5,6 +5,7 @@
 pub mod body_filter;
 pub mod cache_injector;
 pub mod circuit_breaker;
+pub mod copilot_optimizer;
 pub mod error;
 pub mod error_mapper;
 pub(crate) mod failover_switch;