Add a summarize tool

[iiivanova]

Summary

Add a summarize() tool that turns a long analysis, notebook, or report into a short executive summary.

Motivation

The existing tools (evaluate, plan, redteam, ask) help with analysis quality and planning. A summarize tool fills the gap for communicating findings - turning technical output into something a non-technical audience can act on.

Expected usage

from bridgekit import summarize

text = """
[long analysis writeup or notebook output]
"""

print(summarize(text))

Optional parameters:

• audience - e.g. "VP of Marketing", "board", "engineering team" (defaults to general business audience)
• provider, model, system_prompt - consistent with other tools

Implementation notes

• Follow the same pattern as existing tools in bridgekit/
• Single public function in bridgekit/summarize.py, exported from __init__.py
• Add tests under tests/test_summarize.py
• Document in README with example and sample output