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-pager-render/src/link_opener.rs

@@ -214,6 +214,7 @@ pub fn open_url_if_safe(url: &str, filter: SchemeFilter) -> bool {
 mod tests {
     use super::*;
 
+    #[cfg(not(target_os = "windows"))]
     #[test]
     fn open_path_command_passes_path_as_a_single_arg() {
         // Path with spaces must be one argument, never shell-interpolated.

crates/codegen/kigi-pager-render/src/terminal/probe.rs

@@ -10,6 +10,7 @@
 //!   portable re-injection exists (TIOCSTI is blocked); accepted loss.
 
 use std::io::Write;
+#[cfg(unix)]
 use std::time::Duration;
 
 /// Bounds the reply buffer against terminals that stream without a terminator.