Use build args for git user config

Change git configuration to use Docker build args instead of .env file,
simplifying setup and improving security. This change:
- Removes git config from .env and startup.sh
- Adds GIT_USER_NAME and GIT_USER_EMAIL build args
- Updates documentation for new git config approach
- Improves task logging requirements in CLAUDE.md

The build arg approach provides better isolation and ensures git config is
properly set during image build rather than container runtime.

scripts/startup.sh

@@ -34,12 +34,7 @@ else
     echo "No Twilio credentials found - SMS notifications disabled"
 fi
 
-# Configure git for the mounted workspace
-if [ -n "$GIT_USER_NAME" ] && [ -n "$GIT_USER_EMAIL" ]; then
-    echo "✓ Configuring git: $GIT_USER_NAME <$GIT_USER_EMAIL>"
-    git config --global user.name "$GIT_USER_NAME"
-    git config --global user.email "$GIT_USER_EMAIL"
-fi
+# Git configuration is handled during Docker build from host git config
 
 # Start Claude Code with permissions bypass
 echo "Starting Claude Code..."