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.
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