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/models/config.py

@@ -73,11 +73,19 @@ class ShellToolConfig(BaseModel):
     max_output_bytes: int = Field(default=65536, description="Max output capture size in bytes")
 
 
+class FileCacheConfig(BaseModel):
+    """File cache configuration."""
+
+    enabled: bool = Field(default=True, description="Enable file content caching")
+    max_entries: int = Field(default=128, description="Maximum cached file entries (LRU eviction)")
+
+
 class FilesystemToolConfig(BaseModel):
     """Filesystem tool limits."""
 
     max_file_size_bytes: int = Field(default=1_048_576, description="Max file size for read/write")
     binary_detection: bool = Field(default=True, description="Detect and reject binary files")
+    cache: FileCacheConfig = Field(default_factory=FileCacheConfig, description="File cache settings")
 
 
 class ToolsConfig(BaseModel):