fix(auth): unique tmp per auth.json write; tighten loose credential perms

Port of upstream grok-build 0.2.113-window fixes:
- write_auth_json_atomic tmp path gains a monotonic sequence next to the
  pid, so two concurrent in-process writers (background mint + proactive
  refresher) cannot share one tmp path and tear each other's bytes; a tmp
  write that fails partway is reclaimed instead of orphaned.
- New secure_file::ensure_owner_only_permissions (0o600 / Windows ACL)
  tightens pre-existing world-readable credential files on read, backup,
  write, replace, and restore. Best-effort with warns after durable
  content — a chmod-only failure must not masquerade as a failed write —
  and propagating in restore_prior_bytes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSy9S31pZNWXtRZNVJ1sDf

crates/codegen/kigi-shell-base/src/util/secure_file.rs

@@ -26,7 +26,7 @@ use std::io::{self, Write};
 use std::path::Path;
 
 #[cfg(unix)]
-use std::os::unix::fs::OpenOptionsExt;
+use std::os::unix::fs::{OpenOptionsExt, PermissionsExt};
 
 /// Creates or opens a file with secure permissions (owner read/write only).
 ///
@@ -58,17 +58,20 @@ pub fn write_secure_file(path: &Path, contents: &[u8]) -> io::Result<()> {
     file.write_all(contents)?;
     file.flush()?;
 
-    // On Windows, we need to set permissions after file creation
-    #[cfg(windows)]
-    {
-        set_windows_secure_permissions(path)?;
-    }
+    // Re-assert owner-only bits: `OpenOptions::mode` only applies on create,
+    // so an existing world-readable file would otherwise keep open perms.
+    ensure_owner_only_permissions(path)?;
 
     Ok(())
 }
 
 /// Opens a file for writing with secure permissions set during creation (Unix)
 /// or prepares it for permission setting after creation (Windows).
+///
+/// Callers that write secret material should also call
+/// [`ensure_owner_only_permissions`] after the write (or use
+/// [`write_secure_file`]), because `mode(0o600)` only applies when the file
+/// is newly created — not when truncating an existing path.
 pub fn open_secure_file(path: &Path) -> io::Result<File> {
     let mut options = OpenOptions::new();
     options.truncate(true).write(true).create(true);
@@ -82,6 +85,44 @@ pub fn open_secure_file(path: &Path) -> io::Result<File> {
     options.open(path)
 }
 
+/// Ensure `path` is owner-read/write only (Unix `0o600` / Windows user ACL).
+///
+/// Best-effort on missing files (`NotFound` is ignored). Other errors
+/// propagate so callers can fail closed when tightening a secret store.
+///
+/// Use on **load** of credential files so a hand-copied or restored
+/// world-readable `auth.json` is tightened before the process continues.
+pub fn ensure_owner_only_permissions(path: &Path) -> io::Result<()> {
+    match ensure_owner_only_permissions_inner(path) {
+        Ok(()) => Ok(()),
+        Err(e) if e.kind() == io::ErrorKind::NotFound => Ok(()),
+        Err(e) => Err(e),
+    }
+}
+
+fn ensure_owner_only_permissions_inner(path: &Path) -> io::Result<()> {
+    #[cfg(unix)]
+    {
+        let metadata = std::fs::metadata(path)?;
+        let mode = metadata.permissions().mode();
+        if mode & 0o777 != 0o600 {
+            let mut perms = metadata.permissions();
+            perms.set_mode(0o600);
+            std::fs::set_permissions(path, perms)?;
+        }
+        Ok(())
+    }
+    #[cfg(windows)]
+    {
+        set_windows_secure_permissions(path)
+    }
+    #[cfg(not(any(unix, windows)))]
+    {
+        let _ = path;
+        Ok(())
+    }
+}
+
 /// Sets Windows-specific secure permissions on a file.
 ///
 /// This function modifies the file's ACL to:
@@ -219,8 +260,6 @@ mod tests {
     #[cfg(unix)]
     #[test]
     fn test_unix_permissions() {
-        use std::os::unix::fs::PermissionsExt;
-
         let temp_dir = tempfile::tempdir().unwrap();
         let file_path = temp_dir.path().join("test_perms.txt");
 
@@ -231,4 +270,43 @@ mod tests {
         // Check that only owner has read/write (0o600), ignoring file type bits
         assert_eq!(mode & 0o777, 0o600);
     }
+
+    #[cfg(unix)]
+    #[test]
+    fn ensure_owner_only_tightens_world_readable_file() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let file_path = temp_dir.path().join("loose.txt");
+        fs::write(&file_path, b"secret").unwrap();
+        let mut loose = fs::metadata(&file_path).unwrap().permissions();
+        loose.set_mode(0o644);
+        fs::set_permissions(&file_path, loose).unwrap();
+        assert_eq!(
+            fs::metadata(&file_path).unwrap().permissions().mode() & 0o777,
+            0o644
+        );
+
+        ensure_owner_only_permissions(&file_path).unwrap();
+        assert_eq!(
+            fs::metadata(&file_path).unwrap().permissions().mode() & 0o777,
+            0o600
+        );
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn write_secure_file_tightens_existing_world_readable_file() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let file_path = temp_dir.path().join("existing.txt");
+        fs::write(&file_path, b"old").unwrap();
+        let mut loose = fs::metadata(&file_path).unwrap().permissions();
+        loose.set_mode(0o666);
+        fs::set_permissions(&file_path, loose).unwrap();
+
+        write_secure_file(&file_path, b"new secret").unwrap();
+        assert_eq!(
+            fs::metadata(&file_path).unwrap().permissions().mode() & 0o777,
+            0o600
+        );
+        assert_eq!(fs::read_to_string(&file_path).unwrap(), "new secret");
+    }
 }