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/install.rs

@@ -6,7 +6,7 @@
 //! - UAC elevation with graceful fallback
 
 use anyhow::{anyhow, Result};
-use tracing::{error, info, warn};
+use tracing::{info, warn};
 
 #[cfg(windows)]
 use windows::{