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-tools/src/computer/local/terminal.rs

@@ -2653,7 +2653,7 @@ fn spawn_shell_command(
     };
 
     #[cfg(not(unix))]
-    let mut build_cmd = |with_breakaway: bool| {
+    let build_cmd = |with_breakaway: bool| {
         use windows::Win32::System::Threading::{
             CREATE_BREAKAWAY_FROM_JOB, CREATE_NEW_PROCESS_GROUP, CREATE_NO_WINDOW,
         };