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/startup.rs

@@ -3,7 +3,7 @@
 //! Handles adding/removing the agent from Windows startup.
 
 use anyhow::Result;
-use tracing::{error, info, warn};
+use tracing::{info, warn};
 
 #[cfg(windows)]
 use windows::core::PCWSTR;
@@ -58,7 +58,10 @@ pub fn add_to_startup() -> Result<()> {
             anyhow::bail!("Failed to open registry key: {:?}", result);
         }
 
-        let hkey_raw = std::mem::transmute::<_, windows::Win32::System::Registry::HKEY>(hkey);
+        let hkey_raw = std::mem::transmute::<
+            windows::Win32::Foundation::HANDLE,
+            windows::Win32::System::Registry::HKEY,
+        >(hkey);
 
         // Set the value
         let data_bytes =
@@ -116,7 +119,10 @@ pub fn remove_from_startup() -> Result<()> {
             return Ok(()); // Not an error if key doesn't exist
         }
 
-        let hkey_raw = std::mem::transmute::<_, windows::Win32::System::Registry::HKEY>(hkey);
+        let hkey_raw = std::mem::transmute::<
+            windows::Win32::Foundation::HANDLE,
+            windows::Win32::System::Registry::HKEY,
+        >(hkey);
 
         let delete_result = RegDeleteValueW(hkey_raw, PCWSTR(value_name.as_ptr()));
 
@@ -180,6 +186,8 @@ pub fn uninstall() -> Result<()> {
 
 /// Install the SAS service if the binary is available
 /// This allows the agent to send Ctrl+Alt+Del even without SYSTEM privileges
+// Not yet wired into the CLI; retained as the SAS service management API.
+#[allow(dead_code)]
 #[cfg(windows)]
 pub fn install_sas_service() -> Result<()> {
     info!("Attempting to install SAS service...");
@@ -230,6 +238,8 @@ pub fn install_sas_service() -> Result<()> {
 }
 
 /// Uninstall the SAS service
+// Not yet wired into the CLI; retained as the SAS service management API.
+#[allow(dead_code)]
 #[cfg(windows)]
 pub fn uninstall_sas_service() -> Result<()> {
     info!("Attempting to uninstall SAS service...");
@@ -244,16 +254,14 @@ pub fn uninstall_sas_service() -> Result<()> {
         )),
     ];
 
-    for path_opt in paths.iter() {
-        if let Some(ref path) = path_opt {
-            if path.exists() {
-                let output = std::process::Command::new(path).arg("uninstall").output();
+    for path in paths.iter().flatten() {
+        if path.exists() {
+            let output = std::process::Command::new(path).arg("uninstall").output();
 
-                if let Ok(result) = output {
-                    if result.status.success() {
-                        info!("SAS service uninstalled successfully");
-                        return Ok(());
-                    }
+            if let Ok(result) = output {
+                if result.status.success() {
+                    info!("SAS service uninstalled successfully");
+                    return Ok(());
                 }
             }
         }
@@ -264,6 +272,8 @@ pub fn uninstall_sas_service() -> Result<()> {
 }
 
 /// Check if the SAS service is installed and running
+// Not yet wired into the CLI; retained as the SAS service management API.
+#[allow(dead_code)]
 #[cfg(windows)]
 pub fn check_sas_service() -> bool {
     use crate::sas_client;