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/storage/summary_write.rs

@@ -230,7 +230,10 @@ fn write_summary_atomic(summary_path: &Path, summary: &Summary) -> io::Result<()
         .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
     let tmp = summary_path.with_extension("json.tmp");
     std::fs::write(&tmp, &bytes)?;
-    std::fs::rename(&tmp, summary_path)
+    // Windows-safe replace: this is the write that persists a session's
+    // CURRENT MODEL — a bare rename made a switched model silently revert
+    // on resume whenever AV/indexer held summary.json open on Windows.
+    crate::util::fs::replace_file(&tmp, summary_path)
 }
 
 #[cfg(test)]