Bugs fixed:
- filesystem.py: add missing `import re` (grep was broken in production)
- image_view.py: consistent SVG rejection message for URL and file paths
- store/__init__.py: normalize user_id None→'' to prevent duplicate rows
  in unique constraint for anonymous sessions; add DDL migration for
  existing NULL values

Tests:
- Add 10 unit tests for filesystem copy, grep, diff operations

Documentation:
- agent.md: document streaming guard wrapper, system prompt caching,
  ContextVar restoration in subagents
- tools.md: document middleware hooks
- websocket.md: document image upload limits and concurrent run guard
- store.md: document user_id normalization
- mechanics.md: mark newly-documented mechanics as documented

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>