Add summarize_on_exit option for mini agent

[Chesars]

Summary

Adds optional summarize_on_exit config to mini agent.

When enabled, the model generates a summary of changes before the exit prompt.

Changes:

• Added summarize_on_exit field to InteractiveAgentConfig (default: false)
• Implemented _summarize_changes() method that queries the model for a summary
• Summary displays before the exit confirmation prompt
• Added config example in mini.yaml (commented out by default)

Usage:
Set summarize_on_exit: true in any config that uses InteractiveAgent (mini.yaml, github_issue.yaml, etc.)

Closes #293

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/minisweagent/agents/interactive.py	100.00% <100.00%> (ø)
tests/agents/test_interactive.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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