fix(agent): catch PermissionError in subdirectory hint discovery

Wrap is_dir() in _is_valid_subdir() and is_file() in
_load_hints_for_directory() with OSError handlers so that
inaccessible directories (e.g. /root from a non-root Daytona
host user) are silently skipped instead of crashing the agent.

The existing PermissionError PRs for prompt_builder.py (#6247,
#6321, #6355) do not cover subdirectory_hints.py, which was
identified as a separate crash path in the #6214 comments.

Ref: #6214

agent/subdirectory_hints.py

@@ -159,7 +159,10 @@ class SubdirectoryHintTracker:
 
     def _is_valid_subdir(self, path: Path) -> bool:
         """Check if path is a valid directory to scan for hints."""
-        if not path.is_dir():
+        try:
+            if not path.is_dir():
+                return False
+        except OSError:
             return False
         if path in self._loaded_dirs:
             return False
@@ -172,7 +175,10 @@ class SubdirectoryHintTracker:
         found_hints = []
         for filename in _HINT_FILENAMES:
             hint_path = directory / filename
-            if not hint_path.is_file():
+            try:
+                if not hint_path.is_file():
+                    continue
+            except OSError:
                 continue
             try:
                 content = hint_path.read_text(encoding="utf-8").strip()