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/session/mod.rs

@@ -11,7 +11,7 @@ use windows::Win32::System::Console::{AllocConsole, GetConsoleWindow};
 #[cfg(windows)]
 use windows::Win32::UI::WindowsAndMessaging::{ShowWindow, SW_SHOW};
 
-use crate::capture::{self, Capturer, Display};
+use crate::capture::{self, Capturer};
 use crate::chat::{ChatController, ChatMessage as ChatMsg};
 use crate::config::Config;
 use crate::encoder::{self, Encoder};
@@ -22,7 +22,7 @@ use crate::input::InputController;
 fn show_debug_console() {
     unsafe {
         let hwnd = GetConsoleWindow();
-        if hwnd.0 == std::ptr::null_mut() {
+        if hwnd.0.is_null() {
             let _ = AllocConsole();
             tracing::info!("Debug console window opened");
         } else {
@@ -130,7 +130,7 @@ impl SessionManager {
 
         // Get primary display with panic protection
         tracing::debug!("Enumerating displays...");
-        let primary_display = match std::panic::catch_unwind(|| capture::primary_display()) {
+        let primary_display = match std::panic::catch_unwind(capture::primary_display) {
             Ok(result) => result?,
             Err(e) => {
                 tracing::error!("Panic during display enumeration: {:?}", e);
@@ -623,11 +623,8 @@ impl SessionManager {
             Some(message::Payload::SpecialKey(special)) => {
                 if let Some(input) = self.input.as_mut() {
                     use crate::proto::SpecialKey;
-                    match SpecialKey::try_from(special.key).ok() {
-                        Some(SpecialKey::CtrlAltDel) => {
-                            input.send_ctrl_alt_del()?;
-                        }
-                        _ => {}
+                    if let Ok(SpecialKey::CtrlAltDel) = SpecialKey::try_from(special.key) {
+                        input.send_ctrl_alt_del()?;
                     }
                 }
             }