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>

config/config.yaml

@@ -68,6 +68,9 @@ tools:
   filesystem:
     max_file_size_bytes: 1048576  # 1 MB
     binary_detection: true
+    cache:
+      enabled: true
+      max_entries: 128
 
 session:
   session_dir: ".sneakycode/sessions"