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/util/config/campaigns.rs

@@ -114,9 +114,7 @@ fn dismiss_campaign_ids_at(
     let nonce = DISMISS_TMP_NONCE.fetch_add(1, Ordering::Relaxed);
     let tmp = path.with_extension(format!("json.{}.{}.tmp", std::process::id(), nonce));
     std::fs::write(&tmp, &json)?;
-    std::fs::rename(&tmp, &path).inspect_err(|_| {
-        let _ = std::fs::remove_file(&tmp);
-    })
+    crate::util::fs::replace_file(&tmp, &path)
 }
 
 /// `KIGI_CAMPAIGNS_OVERRIDE` JSON array replaces all sources (`[]` = none; beats

crates/codegen/kigi-shell/src/util/config/mcp.rs

@@ -367,7 +367,9 @@ pub async fn save_mcp_disabled_tools(server_name: &str, disabled_tools: &[String
         let _ = tokio::fs::create_dir_all(parent).await;
     }
     tokio::fs::write(&tmp, &toml_str).await?;
-    tokio::fs::rename(&tmp, &path).await?;
+    tokio::task::spawn_blocking(move || crate::util::fs::replace_file(&tmp, &path))
+        .await
+        .map_err(std::io::Error::other)??;
     Ok(())
 }
 
@@ -419,7 +421,9 @@ pub async fn save_mcp_server_enabled(server_name: &str, enabled: bool) -> Result
         let _ = tokio::fs::create_dir_all(parent).await;
     }
     tokio::fs::write(&tmp, &toml_str).await?;
-    tokio::fs::rename(&tmp, &path).await?;
+    tokio::task::spawn_blocking(move || crate::util::fs::replace_file(&tmp, &path))
+        .await
+        .map_err(std::io::Error::other)??;
     Ok(())
 }
 
@@ -476,7 +480,10 @@ pub async fn save_mcp_server_config_at(
         let _ = tokio::fs::create_dir_all(parent).await;
     }
     tokio::fs::write(&tmp, &toml_str).await?;
-    tokio::fs::rename(&tmp, &path).await?;
+    let dest = path.to_path_buf();
+    tokio::task::spawn_blocking(move || crate::util::fs::replace_file(&tmp, &dest))
+        .await
+        .map_err(std::io::Error::other)??;
     Ok(())
 }
 
@@ -553,7 +560,12 @@ pub async fn delete_mcp_server_config_at(
         let _ = tokio::fs::create_dir_all(parent).await;
     }
     tokio::fs::write(&tmp, &toml_str).await?;
-    tokio::fs::rename(&tmp, &path).await?;
+    {
+        let dest = path.to_path_buf();
+        tokio::task::spawn_blocking(move || crate::util::fs::replace_file(&tmp, &dest))
+            .await
+            .map_err(std::io::Error::other)??;
+    }
 
     // Clean up OAuth credentials for the deleted server.
     if let Ok(mut cred_store) = kigi_mcp::credentials::McpCredentialStore::load_default() {

crates/codegen/kigi-shell/src/util/config/persist.rs

@@ -88,7 +88,12 @@ pub async fn save_config(config: &Config) -> Result<()> {
     }
     let _ = prior_mode;
 
-    tokio::fs::rename(&tmp, &path).await?;
+    // Windows-safe replace (delete-first + retry on sharing violations) —
+    // a bare rename made `/model` persistence silently fail on Windows
+    // whenever AV/indexer/cloud-sync held config.toml open.
+    tokio::task::spawn_blocking(move || crate::util::fs::replace_file(&tmp, &path))
+        .await
+        .map_err(|e| anyhow::anyhow!("config replace task: {e}"))??;
     Ok(())
 }
 
@@ -138,11 +143,8 @@ pub(crate) fn atomic_write_string(path: &std::path::Path, content: &str) -> std:
     }
     let _ = prior_mode;
 
-    if let Err(e) = std::fs::rename(&tmp, path) {
-        let _ = std::fs::remove_file(&tmp);
-        return Err(e);
-    }
-    Ok(())
+    // Windows-safe replace; cleans up the tmp file on failure itself.
+    crate::util::fs::replace_file(&tmp, path)
 }
 
 /// Merge `[toolset.ask_user_question]` into the root table. `[toolset]` is