chore(gates): finish the Windows sweep — zero clippy findings workspace-wide

Second layer of platform gaps the mac/linux-only gate history left:
unix-only test imports/helpers now cfg-gated (mermaid, fast-worktree,
hunk-tracker, pager-render link test, three pty_e2e module decls),
clippy-suggested cleanups in Windows-cfg code (file-utils classifier,
tui console-mode restore, shared clipboard arboard-error flow, tools
terminal closure, workspace capability tail), and `cargo update` of h2
(RUSTSEC-2026-0258) and webbrowser (RUSTSEC-2026-0257) so `cargo deny
check advisories` passes again.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSy9S31pZNWXtRZNVJ1sDf

crates/codegen/kigi-mermaid/src/subprocess.rs

@@ -179,9 +179,12 @@ fn reap_process_group(_child: &Child) {
 #[cfg(test)]
 mod tests {
     use super::*;
+    #[cfg(unix)]
     use std::process::Stdio;
+    #[cfg(unix)]
     use std::time::Instant;
 
+    #[cfg(unix)]
     fn detached(mut cmd: Command) -> Command {
         cmd.stdin(Stdio::null())
             .stdout(Stdio::null())