arrowbricks v6.0.0

• Add an explicit DatabricksClient.query_agent interface returning compact,
  typed Markdown on success and actionable JSON diagnostics on failure.
  Query execution, bounded Arrow rendering and cancellation stay in Rust;
  no required Python dependencies are added.
• Agent output has configurable row, UTF-8 byte and cell limits, with an
  explicit completeness footer. It stops fetching a bounded prefix instead
  of downloading the whole result to count it. Existing fetch APIs are unchanged.
• Add the arrowbricks CLI and python -m arrowbricks, with compact Markdown
  on stdout, JSON diagnostics on stderr, and SQL arguments, files and stdin.
• Add agent-mode examples, automated coverage on both protocols, and
  reproducible renderer and live SQL warehouse verification.
• Ship skills/arrowbricks/SKILL.md with agent guidance for the CLI and Python
  API, result completeness, budgets and JSON error recovery.