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/config.rs

@@ -9,7 +9,7 @@ use anyhow::{anyhow, Context, Result};
 use serde::{Deserialize, Serialize};
 use std::io::{Read, Seek, SeekFrom};
 use std::path::PathBuf;
-use tracing::{info, warn};
+use tracing::info;
 use uuid::Uuid;
 
 /// Magic marker for embedded configuration (10 bytes)
@@ -196,7 +196,7 @@ impl Config {
     /// Extract 6-digit support code from filename
     fn extract_support_code(filename: &str) -> Option<String> {
         // Look for patterns like "GuruConnect-123456" or "GuruConnect_123456"
-        for part in filename.split(|c| c == '-' || c == '_' || c == '.') {
+        for part in filename.split(['-', '_', '.']) {
             let trimmed = part.trim();
             if trimmed.len() == 6 && trimmed.chars().all(|c| c.is_ascii_digit()) {
                 return Some(trimmed.to_string());
@@ -435,6 +435,8 @@ impl Config {
 }
 
 /// Example configuration file content
+// Retained for documentation / config-template generation.
+#[allow(dead_code)]
 pub fn example_config() -> &'static str {
     r#"# GuruConnect Agent Configuration