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/input/keyboard.rs

@@ -5,8 +5,7 @@ use anyhow::Result;
 #[cfg(windows)]
 use windows::Win32::UI::Input::KeyboardAndMouse::{
     MapVirtualKeyW, SendInput, INPUT, INPUT_0, INPUT_KEYBOARD, KEYBDINPUT, KEYBD_EVENT_FLAGS,
-    KEYEVENTF_EXTENDEDKEY, KEYEVENTF_KEYUP, KEYEVENTF_SCANCODE, KEYEVENTF_UNICODE,
-    MAPVK_VK_TO_VSC_EX,
+    KEYEVENTF_EXTENDEDKEY, KEYEVENTF_KEYUP, KEYEVENTF_UNICODE, MAPVK_VK_TO_VSC_EX,
 };
 
 /// Keyboard input controller
@@ -16,6 +15,8 @@ pub struct KeyboardController {
     modifiers: ModifierState,
 }
 
+// Modifier tracking is not yet wired into key dispatch.
+#[allow(dead_code)]
 #[derive(Default)]
 struct ModifierState {
     ctrl: bool,
@@ -78,6 +79,7 @@ impl KeyboardController {
     }
 
     /// Type a unicode character
+    #[allow(dead_code)]
     #[cfg(windows)]
     pub fn type_char(&mut self, ch: char) -> Result<()> {
         let mut inputs = Vec::new();
@@ -119,6 +121,7 @@ impl KeyboardController {
     }
 
     /// Type a string of text
+    #[allow(dead_code)]
     #[cfg(windows)]
     pub fn type_string(&mut self, text: &str) -> Result<()> {
         for ch in text.chars() {