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/tools/edit.py

@@ -1,13 +1,17 @@
 """Edit tools: str_replace and patch_apply."""
 
+from __future__ import annotations
+
 import subprocess
 from pathlib import Path
 from typing import Any
 
 from pydantic import BaseModel, Field
 
+from app.models.config import AppConfig
 from app.models.tool_call import ToolResult, ToolResultStatus
 from app.tools.base import BaseTool
+from app.utils.file_cache import FileCache, cached_read_file
 from app.utils.file_helpers import (
     FileSizeError,
     PathSecurityError,
@@ -37,6 +41,12 @@ class StrReplaceTool(BaseTool):
     )
     params_model = StrReplaceParams
 
+    def __init__(
+        self, workspace_root: Path, config: AppConfig, file_cache: FileCache | None = None
+    ) -> None:
+        super().__init__(workspace_root, config)
+        self._file_cache = file_cache
+
     def execute(
         self, *, tool_call_id: str, file_path: str, old_str: str, new_str: str, **kwargs: Any
     ) -> ToolResult:
@@ -44,11 +54,12 @@ class StrReplaceTool(BaseTool):
 
         # Read the file
         try:
-            content = safe_read_file(
+            content = cached_read_file(
                 file_path,
                 self.workspace_root,
                 max_size_bytes=fs_config.max_file_size_bytes,
                 check_binary=fs_config.binary_detection,
+                cache=self._file_cache,
             )
         except PathSecurityError as exc:
             return ToolResult(
@@ -117,8 +128,14 @@ class StrReplaceTool(BaseTool):
             safe_path = resolve_safe_path(file_path, self.workspace_root)
             rel_path = safe_path.relative_to(self.workspace_root)
         except (PathSecurityError, ValueError):
+            safe_path = None
             rel_path = Path(file_path)
 
+        # Pre-warm cache with the new content (we already have it in memory).
+        if self._file_cache is not None and safe_path is not None:
+            self._file_cache.invalidate(safe_path)
+            self._file_cache.put(safe_path, new_content)
+
         return ToolResult(
             tool_call_id=tool_call_id,
             tool_name=self.name,
@@ -144,6 +161,12 @@ class PatchApplyTool(BaseTool):
     )
     params_model = PatchApplyParams
 
+    def __init__(
+        self, workspace_root: Path, config: AppConfig, file_cache: FileCache | None = None
+    ) -> None:
+        super().__init__(workspace_root, config)
+        self._file_cache = file_cache
+
     def execute(self, *, tool_call_id: str, file_path: str, patch: str, **kwargs: Any) -> ToolResult:
         try:
             safe_path = resolve_safe_path(file_path, self.workspace_root)
@@ -195,6 +218,9 @@ class PatchApplyTool(BaseTool):
                 error=f"Patch failed (exit {result.returncode}): {result.stderr or result.stdout}",
             )
 
+        if self._file_cache is not None:
+            self._file_cache.invalidate(safe_path)
+
         try:
             rel_path = safe_path.relative_to(self.workspace_root)
         except ValueError: