fix(packaging): add [build-system] to darnit-mcp root so it builds

[Marc-cn]

release.yml runs uv build --package darnit-mcp, which fails because the root pyproject (name = darnit-mcp) has no build-system. hatchling with bypass-selection = true builds the dependency-only wheel correctly.

Summary

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Framework Changes Checklist

If this PR modifies the darnit framework (packages/darnit/):

• Updated framework spec (openspec/specs/framework-design/spec.md) if behavior changed
• Ran uv run python scripts/validate_sync.py --verbose and it passes
• Ran uv run python scripts/generate_docs.py and committed any doc changes

Control/TOML Changes Checklist

If this PR modifies controls or TOML configuration:

• Control metadata defined in TOML (not Python code)
• SARIF fields (description, severity, help_url) included where appropriate
• Ran validation to confirm TOML schema compliance

Testing

• Tests pass locally (uv run pytest tests/ -v)
• Added tests for new functionality (if applicable)
• Linting passes (uv run ruff check .)

Additional Notes

Per @mlieberman85's request on #261.