chore: drop dead code and the stale wireguard-tools dependency [P2]

wireguard-tools was still declared in Depends and in the runtime package list installed by
tools/build.sh --prepare-runtime-env. Since peer discovery moved to docker exec, no code
path invokes a host-side wg, and check_host_tools stopped requiring it. The declaration
was therefore installing a package the harness never calls and misrepresenting the actual
architecture to anyone reading the control file. Removed from both places.

Three items were carrying #[allow(dead_code)], which is a way of silencing the compiler
about a real question rather than answering it:

- SysctlTuningStatus.desired duplicated data that is already in .current - every
  SysctlValue holds both the desired and the current value, so the second Vec was the same
  information cloned on every inspect call and read by nobody;
- SysctlValue.managed was always set to true and never read. Ownership is decided per file
  by the marker, not per key, so the field encoded nothing;
- unix_now() in the reboot scheduler, along with the SystemTime and UNIX_EPOCH imports it
  needed. The module works in chrono NaiveDateTime throughout.

All three are removed rather than annotated. The workspace now builds clean under
-D warnings with no dead_code exceptions anywhere.

cargo test --workspace: 158 + 4 passed. clippy -D warnings clean.

reboot_module/src/main.rs

@@ -1,10 +1,4 @@
-use std::{
-    fs,
-    path::PathBuf,
-    process::Command,
-    thread,
-    time::{Duration as StdDuration, SystemTime, UNIX_EPOCH},
-};
+use std::{fs, path::PathBuf, process::Command, thread, time::Duration as StdDuration};
 
 use anyhow::Context;
 use chrono::{Duration, NaiveDateTime, NaiveTime, Timelike, Utc};
@@ -188,14 +182,6 @@ fn parse_hhmm(v: &str) -> anyhow::Result<NaiveTime> {
     NaiveTime::from_hms_opt(h, m, 0).ok_or_else(|| anyhow::anyhow!("bad time"))
 }
 
-#[allow(dead_code)]
-fn unix_now() -> u64 {
-    SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .unwrap_or_default()
-        .as_secs()
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;