fix(fs): Windows-safe atomic replace everywhere — model switch now sticks

Root cause of 'model+effort switch works on Mac, not on Windows': the
switch APPLIES in-session (the dispatch/apply chain is platform-identical,
verified adversarially) but its persistence never sticks on Windows.
Every tmp+rename atomic write except auth/storage.rs committed with a
bare fs::rename, and Windows MoveFileExW(REPLACE_EXISTING) fails with a
sharing violation whenever AV/search-indexer/cloud-sync transiently holds
the destination open. Consequences: [models].default never persisted
(next launch = original model), the session summary's current model never
persisted (resume = original model), and the models cache went silently
stale (all its write errors were swallowed).

- New kigi_shell_base::util::fs::replace_file — THE commit step for
  tmp+rename: plain rename on Unix; on Windows delete-first + two short
  backoffs (the pattern auth/storage.rs shipped first), tmp cleaned on
  failure, error always returned. Windows branch type-checked against
  x86_64-pc-windows-msvc.
- Adopted at every replace site: config.toml (save_config /
  atomic_write_string / mcp saves), models cache (plus unique tmp
  suffixes and tracing::warn on failure — writes were fully silent),
  session storage (summary/current-model, jsonl, plan/signals/
  announcement/goal/graph state), auth.json, active-sessions registry,
  prompt history, claude/kimi import, campaigns state, goal artifacts.
  Directory-move renames (worktree pool, corrupt-file backups) keep
  plain rename — their destinations don't pre-exist.

Verified: kigi-shell + kigi-shell-base 5318 tests green, clippy clean,
msvc-target check of the new cfg(windows) code clean.

crates/codegen/kigi-shell/src/session/goal_tracker.rs

@@ -895,7 +895,8 @@ impl GoalTracker {
         };
         let dest = goal_dir.join(name);
         let _ = std::fs::create_dir_all(&goal_dir);
-        if std::fs::rename(&src, &dest).is_ok() || copy_no_follow(&src, &dest).is_ok() {
+        if crate::util::fs::replace_file(&src, &dest).is_ok() || copy_no_follow(&src, &dest).is_ok()
+        {
             append_skeptic_reports(&scratch_root, &dest);
             o.last_classifier_details_path = Some(dest.to_string_lossy().into_owned());
         }