Retire the stale slash-command backlog item without breaking verification

ROADMAP #39 was stale: current main already hides the unimplemented slash
commands from the help/completion surfaces that triggered the original report,
so the backlog entry should be marked done with current evidence instead of
staying open forever.

While rerunning the user's required Rust verification gates on the exact commit
we planned to push, clippy exposed duplicate and unused imports in the plugin
state-isolation files. Folding those cleanup fixes into the same closeout keeps
the proof honest and restores a green workspace before the backlog retirement
lands.

Constraint: User required fresh cargo fmt, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace before push
Rejected: Push the roadmap-only closeout without fixing the workspace | would violate the required verification gate and leave main red
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Re-run the full Rust workspace gates on the exact commit you intend to push when retiring stale roadmap items
Tested: cargo fmt; cargo clippy --workspace --all-targets -- -D warnings; cargo test --workspace
Not-tested: No manual interactive REPL completion/help smoke test beyond the existing automated coverage

rust/crates/plugins/src/lib.rs

@@ -4,7 +4,6 @@ pub mod test_isolation;
 
 use std::collections::{BTreeMap, BTreeSet};
 use std::fmt::{Display, Formatter};
-use std::sync::atomic::{AtomicU64, Ordering};
 use std::fs;
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};

rust/crates/plugins/src/test_isolation.rs

@@ -2,7 +2,6 @@
 // ROADMAP #41: Stop ambient plugin state from skewing CLI regression checks
 
 use std::env;
-use std::fs;
 use std::path::PathBuf;
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::Mutex;