style(agent): clear 77 pre-existing clippy -D warnings

CI never ran clippy on the agent crate (the build-server clippy job is
Linux-only and can't compile the Windows agent; build-agent only runs cargo
build), so 77 clippy -D-warnings errors had accumulated. Behavior-preserving
cleanup, code-reviewed APPROVED, locally verified (cargo clippy --workspace
--all-targets --all-features -- -D warnings exits 0; cargo test --workspace =
57 passed).

- let _ = on Win32 resource-teardown BOOL returns (gdi.rs); fallible
  BitBlt/GetDIBits stay error-handled
- removed unused imports/vars; idiom fixes (div_ceil, is_null, transmute
  annotations, match collapsing, useless_conversion)
- #[allow(dead_code)] + comment on genuine Task-6/7 scaffolding (vk consts,
  SpecialKey emission, SAS mgmt API, modifier tracking, GDI frame-diff fields)
- Cargo.lock: cargo pruned ~147 stale transitive entries (no version changes)

Follow-up: add cargo clippy -D warnings to the build-agent CI job so the agent
crate stays clippy-clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/src/chat/mod.rs

@@ -6,17 +6,13 @@
 use std::sync::mpsc::{self, Receiver, Sender};
 use std::sync::{Arc, Mutex};
 use std::thread;
-use tracing::{error, info, warn};
+use tracing::info;
+#[cfg(not(windows))]
+use tracing::warn;
 
 #[cfg(windows)]
 use windows::core::PCWSTR;
 #[cfg(windows)]
-use windows::Win32::Foundation::*;
-#[cfg(windows)]
-use windows::Win32::Graphics::Gdi::*;
-#[cfg(windows)]
-use windows::Win32::System::LibraryLoader::GetModuleHandleW;
-#[cfg(windows)]
 use windows::Win32::UI::WindowsAndMessaging::*;
 
 /// A chat message
@@ -29,11 +25,15 @@ pub struct ChatMessage {
 }
 
 /// Commands that can be sent to the chat window
+// Show/Hide/Close are part of the chat control API but not yet driven by the session loop.
 #[derive(Debug)]
 pub enum ChatCommand {
+    #[allow(dead_code)]
     Show,
+    #[allow(dead_code)]
     Hide,
     AddMessage(ChatMessage),
+    #[allow(dead_code)]
     Close,
 }
 
@@ -69,11 +69,13 @@ impl ChatController {
     }
 
     /// Show the chat window
+    #[allow(dead_code)]
     pub fn show(&self) {
         let _ = self.command_tx.send(ChatCommand::Show);
     }
 
     /// Hide the chat window
+    #[allow(dead_code)]
     pub fn hide(&self) {
         let _ = self.command_tx.send(ChatCommand::Hide);
     }
@@ -93,16 +95,14 @@ impl ChatController {
     }
 
     /// Close the chat window
+    #[allow(dead_code)]
     pub fn close(&self) {
         let _ = self.command_tx.send(ChatCommand::Close);
     }
 }
 
 #[cfg(windows)]
-fn run_chat_window(command_rx: Receiver<ChatCommand>, message_tx: Sender<ChatMessage>) {
-    use std::ffi::OsStr;
-    use std::os::windows::ffi::OsStrExt;
-
+fn run_chat_window(command_rx: Receiver<ChatCommand>, _message_tx: Sender<ChatMessage>) {
     info!("Starting chat window thread");
 
     // For now, we'll use a simple message box approach