Improve first-start default worker templates

[pmbstyle]

Summary

• align first-start worker template docs and sync behavior with the workspace_templates -> workspace/workers model
• upgrade default worker prompts and tool sets to better match the current runtime and fetch stack
• add first-start worker specializations for repo research, file editing, bug investigation, and implementation orchestration

Testing

• uv run pytest tests/test_configure_workspace_bootstrap.py tests/test_worker_router.py tests/test_worker_child_spawning.py tests/test_parallel_worker_orchestration.py tests/test_worker_template_security.py