Add guarded implement-all command

[KKranthi6881]

Summary

• add /dbt.implement-all command template for sequential multi-task implementation
• enforce validation and commit checkpoints after each task
• document stop conditions and no-auto-merge safety rules
• update README, CLAUDE template, tutorials, and docs tests

Verification

• uv run --extra dev ruff check src tests
• uv run --extra dev mypy src
• uv run --extra dev pytest
• uv run python -m build
• uv run python -m twine check dist/*