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-fast-worktree/src/copy/cow.rs

@@ -76,6 +76,7 @@ mod tests {
         assert_eq!(std::fs::read(&dest).unwrap(), data);
     }
 
+    #[cfg(unix)]
     #[test]
     fn test_clone_preserves_permissions() {
         use std::os::unix::fs::PermissionsExt;

crates/codegen/kigi-fast-worktree/src/worktree/mod.rs

@@ -37,6 +37,7 @@ pub(crate) fn execute_plan(plan: WorktreePlan) -> Result<CreateWorktreeResult> {
 
 #[cfg(test)]
 mod tests {
+    #[cfg(unix)]
     use super::*;
     use crate::{IgnoredFilesMode, WorkingTreeMode, WorktreeBuilder};
     use kigi_test_utils::git::{git_commit_all, init_git_repo};