feat: add tool-level file cache with LRU eviction and mtime invalidation

Introduces FileCache (OrderedDict LRU, st_mtime_ns validation) to avoid
redundant disk reads and duplicate content in conversation context.
Read tools return a short "[Cached]" message on cache hit instead of
resending unchanged file content, saving tokens. Write/edit/delete tools
invalidate affected paths; str_replace pre-warms the cache after edits.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/ui/app.py

@@ -97,11 +97,20 @@ class SneakyCodeApp(App):
                 self._config.skills, self._config.agent.workspace_root
             )
 
+        # Create file cache if enabled
+        self._file_cache = None
+        fs_cache_cfg = self._config.tools.filesystem.cache
+        if fs_cache_cfg.enabled:
+            from app.utils.file_cache import FileCache
+
+            self._file_cache = FileCache(max_entries=fs_cache_cfg.max_entries)
+
         # Create tool registry (SkillRunner wired after registry exists)
         self._tool_registry = create_default_registry(
             self._config.agent.workspace_root,
             self._config,
             skills_manager=self._skills_manager,
+            file_cache=self._file_cache,
         )
 
         # Create SkillRunner and late-bind it to skill tools