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/capture/gdi.rs

@@ -48,7 +48,7 @@ impl GdiCapturer {
 
             let bitmap = CreateCompatibleBitmap(screen_dc, self.width as i32, self.height as i32);
             if bitmap.is_invalid() {
-                DeleteDC(mem_dc);
+                let _ = DeleteDC(mem_dc);
                 ReleaseDC(HWND::default(), screen_dc);
                 anyhow::bail!("Failed to create compatible bitmap");
             }
@@ -69,8 +69,8 @@ impl GdiCapturer {
                 SRCCOPY,
             ) {
                 SelectObject(mem_dc, old_bitmap);
-                DeleteObject(bitmap);
-                DeleteDC(mem_dc);
+                let _ = DeleteObject(bitmap);
+                let _ = DeleteDC(mem_dc);
                 ReleaseDC(HWND::default(), screen_dc);
                 anyhow::bail!("BitBlt failed: {}", e);
             }
@@ -110,8 +110,8 @@ impl GdiCapturer {
 
             // Cleanup
             SelectObject(mem_dc, old_bitmap);
-            DeleteObject(bitmap);
-            DeleteDC(mem_dc);
+            let _ = DeleteObject(bitmap);
+            let _ = DeleteDC(mem_dc);
             ReleaseDC(HWND::default(), screen_dc);
 
             if lines == 0 {